Implement Supabase Realtime subscription for attestation queue updates

Use supabase.channel('attestation-queue-{chapterId}').onPostgresChanges(event: PostgresChangeEvent.all, schema: 'public', table: 'expense_claims', filter: PostgresChangeFilter(type: FilterType.eq, column: 'chapter_id', value: chapterId), callback: _handleRealtimeEvent).subscribe(). In _handleRealtimeEvent, for INSERT events fetch the full PendingClaim details via a single REST call (the Realtime payload only contains raw columns, not joined claimant name). For UPDATE events check the new.status field: if not 'pending-attestation', remove from local list. Override ref.onDispose() in the AsyncNotifier to call _channel.unsubscribe().

For the backgrounding/foreground reconciliation, use WidgetsBindingObserver in a separate provider that watches AppLifecycleState.resumed and calls ref.invalidate(attestationQueueProvider). Avoid storing the full PendingClaim in Realtime payloads — this keeps payload size minimal and avoids transmitting PII over the WebSocket.

Unit tests with a fake RealtimeChannel that emits synthetic payload events: (1) INSERT payload → new PendingClaim fetched and inserted at correct sort position; (2) UPDATE with status='approved' → claim removed from list; (3) UPDATE with status='pending-attestation' (re-opened) → claim added back; (4) channel disconnect → isConnected flag set false, reconciliation fetch triggered on reconnect; (5) provider disposal → channel.unsubscribe() called exactly once. Use flutter_test. Integration test on a staging Supabase project: open two Dart isolates, submit a claim from isolate B, verify isolate A's notifier receives the INSERT event and queue length increases by 1 within 5 seconds.