refactor: /simplify — 22 fixes from 3-agent code review

Critical:
- Fix MXC URI resolution: all avatars/images now resolve mxc:// to HTTP
- Sync persistence: only write changed rooms, batch message upserts
- lastActivityAt uses room.lastEvent.originServerTs, not creation time

High:
- Shared MatrixAvatar widget replaces 6 duplicate implementations
- CallScreen decodes roomId before LiveKit JWT fetch
- Decline button actually dismisses incoming call overlay
- EventTypes constants replace raw string literals
- LiveKitService uses lazy auth reads, onDispose disconnects

Medium:
- CallController is keepAlive with timer/room cleanup
- authRepository is keepAlive (used from keepAlive notifier)
- StreamController not closed in stopListening (crash fix)
- Index on messages.roomId for query performance
- 400ms debounce on user search
- Static DateFormat in MessageBubble
- Hardcoded strings replaced with AppConfig refs
- Duplicate isDirectMessage field removed from RoomModel
- E2EE profile claim corrected to Phase 3

Shared utilities:
- lib/shared/widgets/matrix_avatar.dart
- lib/shared/utils/mxc_url.dart
- lib/shared/utils/room_preview.dart
- lib/shared/utils/matrix_id.dart

rawJson column removed (unused, caused main-thread jsonEncode)
Schema migrated to v2 with roomId index.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/features/calls/data/matrixrtc_repository.dart

@@ -1,4 +1,4 @@
-// Version: 1.1.0 | Created: 2026-04-01
+// Version: 1.2.0 | Created: 2026-04-01 | Updated: 2026-04-02
 // MatrixRTC repository — handles outgoing call invites and detects incoming
 // calls via m.call.invite events per MSC4143 (MatrixRTC spec).
 //
@@ -13,6 +13,8 @@ import 'package:riverpod_annotation/riverpod_annotation.dart';
 import '../../../core/auth/auth_notifier.dart';
 import '../../../core/auth/auth_state.dart';
 import '../../../core/network/matrix_client.dart';
+import '../../../shared/utils/matrix_id.dart';
+import '../../../shared/utils/mxc_url.dart';
 import '../domain/incoming_call.dart';
 
 part 'matrixrtc_repository.g.dart';
@@ -42,6 +44,11 @@ class MatrixRtcRepository {
   void stopListening() {
     _eventSubscription?.cancel();
     _eventSubscription = null;
+  }
+
+  /// Close the stream controller. Called from ref.onDispose only.
+  void dispose() {
+    stopListening();
     _incomingCallController.close();
   }
 
@@ -59,7 +66,7 @@ class MatrixRtcRepository {
     final callId = 'call_${DateTime.now().millisecondsSinceEpoch}';
 
     await room.sendEvent({
-      'msgtype': 'm.call.invite',
+      'msgtype': EventTypes.CallInvite,
       'call_id': callId,
       'lifetime': 60000, // 60 seconds before invite expires
       'offer': {
@@ -69,12 +76,12 @@ class MatrixRtcRepository {
       'version': '1',
       'invitee': null, // null = invite entire room
       'm.intentional_mentions': {'user_ids': [], 'room': false},
-    }, type: 'm.call.invite');
+    }, type: EventTypes.CallInvite);
   }
 
   void _onEvent(EventUpdate update) {
     if (update.type != EventUpdateType.timeline) return;
-    if (update.content['type'] != 'm.call.invite') return;
+    if (update.content['type'] != EventTypes.CallInvite) return;
 
     final senderId = update.content['sender'] as String?;
     // Ignore our own invites.
@@ -98,8 +105,8 @@ class MatrixRtcRepository {
         roomId: roomId,
         callerId: senderId ?? '',
         callerDisplayName:
-            senderProfile?.displayName ?? senderId?.split(':').first ?? '',
-        callerAvatarUrl: senderProfile?.avatarUrl?.toString(),
+            senderProfile?.displayName ?? senderId?.matrixLocalpart ?? '',
+        callerAvatarUrl: resolveMxcUrl(_client, senderProfile?.avatarUrl),
         isVideo: (content['offer'] != null),
       ),
     );
@@ -114,6 +121,6 @@ MatrixRtcRepository matrixRtcRepository(Ref ref) {
 
   final repo = MatrixRtcRepository(client: client, myUserId: myUserId);
   repo.startListening();
-  ref.onDispose(repo.stopListening);
+  ref.onDispose(repo.dispose);
   return repo;
 }