feat: auto-refresh calendar subscriptions

backend/db.py

@@ -22,6 +22,12 @@ def get_engine():
     return _engine
 
 
+def get_session_factory() -> async_sessionmaker[AsyncSession]:
+    get_engine()
+    assert _session_factory is not None
+    return _session_factory
+
+
 def reset_engine() -> None:
     global _engine, _session_factory
     _engine = None
@@ -29,9 +35,7 @@ def reset_engine() -> None:
 
 
 async def get_db() -> AsyncIterator[AsyncSession]:
-    get_engine()
-    assert _session_factory is not None
-    async with _session_factory() as session:
+    async with get_session_factory()() as session:
         yield session