fix: reduce event expiration to 20s and fix foreground service crash

- Change EXPIRATION_SECONDS from 300 to 20 for all call signaling
  events (offer, answer, ICE, hangup, reject, renegotiate)
- Change MAX_EVENT_AGE_SECONDS from 30 to 20 to match
- Update NIP-AC doc accordingly
- Fix SecurityException crash on SDK 36: phoneCall foreground service
  type requires MANAGE_OWN_CALLS permission. Switch to microphone
  type which only needs RECORD_AUDIO (already granted).

https://claude.ai/code/session_017hZm7yu7CzmcQgZGSaqSXS

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

@@ -61,7 +61,7 @@ class CallManager(
     companion object {
         const val CALL_TIMEOUT_MS = 60_000L // 60 seconds ringing timeout
         const val ENDED_DISPLAY_MS = 2_000L // show "call ended" briefly before resetting
-        const val MAX_EVENT_AGE_SECONDS = 30L // discard signaling events older than this
+        const val MAX_EVENT_AGE_SECONDS = 20L // discard signaling events older than this
     }
 
     private fun isEventTooOld(event: Event): Boolean = TimeUtils.now() - event.createdAt > MAX_EVENT_AGE_SECONDS