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/chat/data/chat_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
 // Chat repository — bridges Matrix SDK timeline to app domain models.
 // Phase 2 additions: sendFile, sendReaction, redactEvent, reply support.
 
@@ -6,6 +6,8 @@ import 'package:matrix/matrix.dart';
 import 'package:riverpod_annotation/riverpod_annotation.dart';
 
 import '../../../core/network/matrix_client.dart';
+import '../../../shared/utils/matrix_id.dart';
+import '../../../shared/utils/mxc_url.dart';
 import '../domain/message_model.dart';
 
 part 'chat_repository.g.dart';
@@ -105,31 +107,21 @@ class ChatRepository {
     final myUserId = _client.userID ?? '';
     final models = <MessageModel>[];
     for (final e in timeline.events) {
-      models.add(await _toModel(e, timeline, myUserId));
+      models.add(_toModel(e, timeline, myUserId));
     }
     return models.reversed.toList();
   }
 
-  Future<MessageModel> _toModel(
-    Event event,
-    Timeline timeline,
-    String myUserId,
-  ) async {
+  MessageModel _toModel(Event event, Timeline timeline, String myUserId) {
     final senderProfile = event.room.unsafeGetUserFromMemoryOrFallback(
       event.senderId,
     );
 
-    // Resolve mxc:// to an authenticated HTTP URL for display.
+    // Resolve mxc:// to an HTTP URL for display.
     final mxcUrl = _extractMxcUrl(event);
     String? resolvedMediaUrl;
     if (mxcUrl != null) {
-      try {
-        final mxcUri = Uri.parse(mxcUrl);
-        final httpUri = await mxcUri.getDownloadUri(_client);
-        resolvedMediaUrl = httpUri.toString();
-      } on Exception {
-        // Leave as null — the bubble will show a broken image indicator.
-      }
+      resolvedMediaUrl = resolveMxcUrl(_client, Uri.parse(mxcUrl));
     }
 
     // Build reactions map: emoji → [senderId, ...]
@@ -156,8 +148,8 @@ class ChatRepository {
       roomId: event.roomId ?? '',
       senderId: event.senderId,
       senderDisplayName:
-          senderProfile.displayName ?? event.senderId.split(':').first,
-      senderAvatarUrl: senderProfile.avatarUrl?.toString(),
+          senderProfile.displayName ?? event.senderId.matrixLocalpart,
+      senderAvatarUrl: resolveMxcUrl(_client, senderProfile.avatarUrl),
       timestamp: event.originServerTs,
       type: _messageType(event),
       body: event.redacted ? null : event.body,