fix(call): serve RTC transports to Element Call over MSC4515 — calls work again

Since the upstream v0.25.0 merge, Element Call in widget mode no longer
reads .well-known for its LiveKit transport; it asks the HOST via the
widget API (org.matrix.msc4515.get_rtc_transports, capability
org.matrix.msc4515.rtc_transports). cinny never granted the capability
nor implemented WidgetDriver.getRtcTransports(), so discovery returned
nothing and every join failed with "Call is not supported"
(MISSING_MATRIX_RTC_TRANSPORT) — the [LocalMembership] Multiple
Transport Errors line in the browser console.

- matrix-widget-api 1.17.0 -> 1.18.0 (adds MSC4515; also changes the
  sendDelayedEvent driver signature, adapted — parent delay ids were
  removed from the draft).
- Grant MSC4515RtcTransports in getCallCapabilities.
- CallWidgetDriver.getRtcTransports(): homeserver /rtc/transports
  (MSC4143) first, then the .well-known org.matrix.msc4143.rtc_foci list
  (what matrix.lotusguild.org advertises today). Unit-tested.

Server side needs no change: the well-known already carries the livekit
focus and the JWT service answers.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/plugins/call/utils.ts

@@ -20,6 +20,10 @@ export function getCallCapabilities(
   capabilities.add(MatrixCapabilities.MSC3846TurnServers);
   capabilities.add(MatrixCapabilities.MSC4157SendDelayedEvent);
   capabilities.add(MatrixCapabilities.MSC4157UpdateDelayedEvent);
+  // MSC4515: Element Call >= 0.22 in widget mode discovers its LiveKit
+  // transport by asking the host (it no longer reads .well-known itself), so
+  // without this capability every call fails with MISSING_MATRIX_RTC_TRANSPORT.
+  capabilities.add(MatrixCapabilities.MSC4515RtcTransports);
   capabilities.add(`org.matrix.msc2762.timeline:${roomId}`);
   capabilities.add(`org.matrix.msc2762.state:${roomId}`);