feat(notes): show stale-relay hint on replaceable events using NIP-66 cache

Surface a soft "this content's relay may be stale" UX cue on addressable
replaceable events (kind:30xxx) when every delivering relay's most recent
NIP-66 kind:30166 Relay Discovery monitor report cached locally is older
than 14 days (or has never been observed).

Read-only: uses only what's already in LocalCache, populated by the
existing RelayInfoNip66FilterSubAssembler. No new network fetches.

Implementation:
- New `StaleRelayHint` composable in `ui/note/elements/`, hooked into
  `NoteBody` after the zap-splits row. Skips quietly for non-addressable
  events, empty relay sets, or any relay still observed within 14 days.
- Pure `isStaleByLatestMonitorReports(latestPerRelay, now, threshold)`
  predicate — `null` (never monitored) and "older than cutoff" both count
  as stale, but a single fresh relay short-circuits the hint to off.
- Reactively tracks the note's relay set via
  `baseNote.flow().relays.stateFlow` so newly-observed relays update the
  hint without recomposition tricks.
- Latest monitor `created_at` per relay is read from `LocalCache` with
  the same `Filter(kinds=[30166], #d=[relay.url], limit=1)` shape used
  on the Relay Information screen.

Out of scope (per issue):
- Auto-refreshing or hiding stale content.
- Heuristics beyond the "all delivering relays stale" check.

Tests: 9 unit tests for the pure predicate covering empty/single/mixed
sets, null-handling, exact-cutoff boundary, and a custom threshold.

Closes #2762.

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/note/NoteCompose.kt

@@ -88,6 +88,7 @@ import com.vitorpamplona.amethyst.ui.note.elements.DisplayReward
 import com.vitorpamplona.amethyst.ui.note.elements.MoreOptionsButton
 import com.vitorpamplona.amethyst.ui.note.elements.Reward
 import com.vitorpamplona.amethyst.ui.note.elements.ShowForkInformation
+import com.vitorpamplona.amethyst.ui.note.elements.StaleRelayHint
 import com.vitorpamplona.amethyst.ui.note.elements.TimeAgo
 import com.vitorpamplona.amethyst.ui.note.types.BadgeDisplay
 import com.vitorpamplona.amethyst.ui.note.types.DisplayBlockedRelayList
@@ -836,6 +837,7 @@ fun NoteBody(
             Spacer(modifier = HalfDoubleVertSpacer)
             DisplayZapSplits(noteEvent, false, accountViewModel, nav)
         }
+        StaleRelayHint(baseNote)
     }
 }
 

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/note/elements/StaleRelayHint.kt

@@ -0,0 +1,163 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.ui.note.elements
+
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.style.TextOverflow
+import androidx.compose.ui.unit.dp
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import com.vitorpamplona.amethyst.R
+import com.vitorpamplona.amethyst.commons.icons.symbols.Icon
+import com.vitorpamplona.amethyst.commons.icons.symbols.MaterialSymbols
+import com.vitorpamplona.amethyst.model.LocalCache
+import com.vitorpamplona.amethyst.model.Note
+import com.vitorpamplona.amethyst.ui.stringRes
+import com.vitorpamplona.amethyst.ui.theme.warningColor
+import com.vitorpamplona.quartz.nip01Core.core.AddressableEvent
+import com.vitorpamplona.quartz.nip01Core.relay.filters.Filter
+import com.vitorpamplona.quartz.nip01Core.relay.normalizer.NormalizedRelayUrl
+import com.vitorpamplona.quartz.nip66RelayMonitor.discovery.RelayDiscoveryEvent
+import com.vitorpamplona.quartz.utils.TimeUtils
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.combine
+import kotlinx.coroutines.flow.flowOf
+import kotlinx.coroutines.flow.map
+
+/**
+ * NIP-66 stale-relay hint for replaceable/addressable events.
+ *
+ * Soft, read-only UX cue: when an addressable event was delivered exclusively
+ * by relays whose latest NIP-66 `kind:30166` Relay Discovery monitor report is
+ * older than [StaleRelayThresholds.DEFAULT_THRESHOLD_SECS] (14 days), render
+ * an inline warning. The cache is populated by `RelayInfoNip66FilterSubAssembler`
+ * (visited via the Relay Information screen). No network is fetched here.
+ *
+ * Out of scope: auto-refreshing or hiding stale content. Heuristics beyond the
+ * "all delivering relays stale" check.
+ */
+object StaleRelayThresholds {
+    /** 14 days, per the issue's behaviour spec. Hardcoded for v1. */
+    const val DEFAULT_THRESHOLD_SECS: Long = 14L * TimeUtils.ONE_DAY
+}
+
+/**
+ * Pure helper: given the latest monitor `created_at` timestamp for each
+ * delivering relay (null when no kind:30166 has been observed for that relay),
+ * decide whether the event should be flagged as stale.
+ *
+ * Returns true iff the set is non-empty AND every entry is null OR older than
+ * `now - thresholdSecs`. The "all stale" rule keeps the hint quiet whenever at
+ * least one delivering relay still has a fresh monitor heartbeat.
+ */
+fun isStaleByLatestMonitorReports(
+    latestPerRelay: List<Long?>,
+    now: Long,
+    thresholdSecs: Long = StaleRelayThresholds.DEFAULT_THRESHOLD_SECS,
+): Boolean {
+    if (latestPerRelay.isEmpty()) return false
+    val cutoff = now - thresholdSecs
+    return latestPerRelay.all { it == null || it < cutoff }
+}
+
+@Composable
+fun StaleRelayHint(
+    baseNote: Note,
+    modifier: Modifier = Modifier,
+) {
+    val event = baseNote.event ?: return
+    if (event !is AddressableEvent) return
+
+    // Reactively track the delivering relay set. Notes pick up extra relays as
+    // they are observed elsewhere; reuse the same flow `RelayListBox` uses so
+    // the hint converges on the latest set.
+    val noteRelays by baseNote
+        .flow()
+        .relays.stateFlow
+        .collectAsStateWithLifecycle()
+
+    val relays = noteRelays.note.relays
+    if (relays.isEmpty()) return
+
+    val combinedFlow: Flow<List<Long?>> =
+        remember(relays) {
+            if (relays.isEmpty()) {
+                flowOf(emptyList())
+            } else {
+                combine(relays.map { latestRelayDiscoveryCreatedAt(it) }) { values ->
+                    values.toList()
+                }
+            }
+        }
+
+    val latestPerRelay by combinedFlow.collectAsState(initial = emptyList())
+
+    if (latestPerRelay.isEmpty()) return
+
+    val isStale = isStaleByLatestMonitorReports(latestPerRelay, TimeUtils.now())
+    if (!isStale) return
+
+    Row(
+        modifier = modifier.padding(top = 4.dp),
+        verticalAlignment = Alignment.CenterVertically,
+    ) {
+        Icon(
+            symbol = MaterialSymbols.Warning,
+            contentDescription = stringRes(id = R.string.stale_relay_hint_icon_description),
+            tint = MaterialTheme.colorScheme.warningColor,
+            modifier = Modifier.size(14.dp),
+        )
+        Text(
+            text = stringRes(id = R.string.stale_relay_hint_label),
+            color = MaterialTheme.colorScheme.warningColor,
+            style = MaterialTheme.typography.labelSmall,
+            maxLines = 2,
+            overflow = TextOverflow.Ellipsis,
+            modifier = Modifier.padding(start = 4.dp),
+        )
+    }
+}
+
+/**
+ * Emits the most recent `kind:30166` `created_at` we have cached for [relay],
+ * or `null` while none has been observed. The underlying `LocalCache` filter
+ * subscription is shared with the Relay Information screen.
+ */
+private fun latestRelayDiscoveryCreatedAt(relay: NormalizedRelayUrl): Flow<Long?> {
+    val filter =
+        Filter(
+            kinds = listOf(RelayDiscoveryEvent.KIND),
+            tags = mapOf("d" to listOf(relay.url)),
+            limit = 1,
+        )
+    return LocalCache.observeEvents<RelayDiscoveryEvent>(filter).map { events ->
+        events.maxOfOrNull { it.createdAt }
+    }
+}

amethyst/src/main/res/values/strings.xml

@@ -168,6 +168,8 @@
     <string name="pronouns">Pronouns</string>
     <string name="ln_address">LN Address</string>
     <string name="ln_url_outdated">LN URL (outdated)</string>
+    <string name="stale_relay_hint_label">Source relays may be stale</string>
+    <string name="stale_relay_hint_icon_description">Stale relay warning</string>
     <string name="download_to_phone">Save to Phone</string>
     <string name="save_to_gallery">Save to Gallery</string>
     <string name="image_saved_to_the_gallery">Image saved to the phone\'s photo gallery</string>

amethyst/src/test/java/com/vitorpamplona/amethyst/ui/note/elements/StaleRelayHintTest.kt

@@ -0,0 +1,116 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.ui.note.elements
+
+import com.vitorpamplona.quartz.utils.TimeUtils
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class StaleRelayHintTest {
+    private val now = 1_730_000_000L
+    private val freshSecsAgo = TimeUtils.ONE_DAY.toLong() // 1 day ago — fresh
+    private val staleSecsAgo = StaleRelayThresholds.DEFAULT_THRESHOLD_SECS + TimeUtils.ONE_DAY // > 14d ago
+
+    @Test
+    fun `empty relay set is never stale`() {
+        assertFalse(isStaleByLatestMonitorReports(emptyList(), now))
+    }
+
+    @Test
+    fun `single fresh relay is not stale`() {
+        assertFalse(
+            isStaleByLatestMonitorReports(listOf(now - freshSecsAgo), now),
+        )
+    }
+
+    @Test
+    fun `single stale relay is stale`() {
+        assertTrue(
+            isStaleByLatestMonitorReports(listOf(now - staleSecsAgo), now),
+        )
+    }
+
+    @Test
+    fun `single never-monitored relay (null) is treated as stale`() {
+        assertTrue(
+            isStaleByLatestMonitorReports(listOf(null), now),
+        )
+    }
+
+    @Test
+    fun `mixed fresh-and-stale set is not stale (any-fresh wins)`() {
+        assertFalse(
+            isStaleByLatestMonitorReports(
+                listOf(now - staleSecsAgo, now - freshSecsAgo),
+                now,
+            ),
+        )
+    }
+
+    @Test
+    fun `mixed null-and-fresh set is not stale (any-fresh wins)`() {
+        assertFalse(
+            isStaleByLatestMonitorReports(
+                listOf(null, now - freshSecsAgo),
+                now,
+            ),
+        )
+    }
+
+    @Test
+    fun `all-stale-or-null set is stale`() {
+        assertTrue(
+            isStaleByLatestMonitorReports(
+                listOf(null, now - staleSecsAgo, now - staleSecsAgo - TimeUtils.ONE_DAY),
+                now,
+            ),
+        )
+    }
+
+    @Test
+    fun `relay exactly at threshold cutoff is considered fresh`() {
+        // cutoff = now - thresholdSecs; helper uses strict `<`, so equal-to-cutoff is fresh
+        val cutoff = now - StaleRelayThresholds.DEFAULT_THRESHOLD_SECS
+        assertFalse(
+            isStaleByLatestMonitorReports(listOf(cutoff), now),
+        )
+    }
+
+    @Test
+    fun `custom threshold is honored`() {
+        // 1-hour threshold: 30 minutes ago is fresh, 2 hours ago is stale.
+        assertFalse(
+            isStaleByLatestMonitorReports(
+                listOf(now - 30 * 60),
+                now,
+                thresholdSecs = 60 * 60,
+            ),
+        )
+        assertTrue(
+            isStaleByLatestMonitorReports(
+                listOf(now - 2 * 60 * 60),
+                now,
+                thresholdSecs = 60 * 60,
+            ),
+        )
+    }
+}