Add session cleanup task and update documentation

- Implement session_cleanup task for removing expired sessions
- Add comprehensive tests for session cleanup functionality
- Update architecture and task documentation
- Integrate cleanup task into application startup

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/startup.py

@@ -19,7 +19,7 @@ from apscheduler.schedulers.asyncio import AsyncIOScheduler  # type: ignore[impo
 from app.db import init_db, open_db
 from app.services import setup_service
 from app.services.geo_cache import GeoCache
-from app.tasks import blocklist_import, geo_cache_flush, geo_re_resolve, health_check, history_sync
+from app.tasks import blocklist_import, geo_cache_flush, geo_re_resolve, health_check, history_sync, session_cleanup
 from app.utils.async_utils import run_blocking
 from app.utils.jail_config import ensure_jail_configs
 from app.utils.runtime_state import set_runtime_settings
@@ -176,6 +176,7 @@ async def startup_shared_resources(
         geo_cache_flush.register(app)
         geo_re_resolve.register(app)
         history_sync.register(app)
+        session_cleanup.register(app)
 
         return http_session, scheduler
     except Exception: