feat: Add presence heartbeat for Matrix online status

- matrix-gateway: POST /internal/matrix/presence/online endpoint
- usePresenceHeartbeat hook with activity tracking
- Auto away after 5 min inactivity
- Offline on page close/visibility change
- Integrated in MatrixChatRoom component

services/memory-orchestrator/backends/__init__.py

@@ -0,0 +1,9 @@
+from .short_term_pg import ShortTermBackend
+from .vector_store_pg import VectorStoreBackend
+from .kb_filesystem import KnowledgeBaseBackend
+
+__all__ = ['ShortTermBackend', 'VectorStoreBackend', 'KnowledgeBaseBackend']
+
+
+
+