feat: switch NIP-AC from GiftWrap with expiration to EphemeralGiftWrap

Use EphemeralGiftWrapEvent (kind 21059) instead of GiftWrapEvent (kind
1059) for WebRTC call signaling. The ephemeral kind signals to relays
that these events are transient and should not be persisted, eliminating
the need for expiration tags on both inner signaling events and outer
wraps.

Changes:
- Remove expiration tags from all 6 call event types (25050-25055)
- Switch WebRtcCallFactory to produce EphemeralGiftWrapEvent wraps
- Update CallManager and CallController publishEvent types
- Update Account.publishCallSignaling signature
- Use CallManager.MAX_EVENT_AGE_SECONDS for staleness checks
- Update NIP-AC spec to document EphemeralGiftWrap usage
- Remove expiration-related tests

https://claude.ai/code/session_014kyBgZx7cNyeUXYWV25M4j

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/call/CallManager.kt

@@ -23,7 +23,7 @@ package com.vitorpamplona.amethyst.commons.call
 import com.vitorpamplona.quartz.nip01Core.core.Event
 import com.vitorpamplona.quartz.nip01Core.core.HexKey
 import com.vitorpamplona.quartz.nip01Core.signers.NostrSigner
-import com.vitorpamplona.quartz.nip59Giftwrap.wraps.GiftWrapEvent
+import com.vitorpamplona.quartz.nip59Giftwrap.wraps.EphemeralGiftWrapEvent
 import com.vitorpamplona.quartz.nipACWebRtcCalls.WebRtcCallFactory
 import com.vitorpamplona.quartz.nipACWebRtcCalls.events.CallAnswerEvent
 import com.vitorpamplona.quartz.nipACWebRtcCalls.events.CallHangupEvent
@@ -46,7 +46,7 @@ class CallManager(
     private val signer: NostrSigner,
     private val scope: CoroutineScope,
     private val isFollowing: (HexKey) -> Boolean,
-    private val publishEvent: (GiftWrapEvent) -> Unit,
+    private val publishEvent: (EphemeralGiftWrapEvent) -> Unit,
 ) {
     private val factory = WebRtcCallFactory()