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/tasks/session_cleanup.py

@@ -0,0 +1,72 @@
+"""Session cleanup background task.
+
+Registers an APScheduler job that periodically removes expired sessions from
+the ``sessions`` table. Without this cleanup, the table grows unbounded over
+months of operation and degrades query performance.
+
+Individual expired sessions are removed on-demand when validated, but the bulk
+cleanup ensures comprehensive pruning at a predictable interval.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import structlog
+
+from app.repositories import session_repo
+from app.tasks.db import task_db
+from app.utils.runtime_state import get_effective_settings
+from app.utils.time_utils import utc_now
+
+if TYPE_CHECKING:
+    from fastapi import FastAPI
+
+    from app.config import Settings
+
+log: structlog.stdlib.BoundLogger = structlog.get_logger()
+
+#: How often the cleanup job fires (seconds).  Configurable tuning constant.
+SESSION_CLEANUP_INTERVAL: int = 6 * 60 * 60  # 6 hours
+
+#: Stable APScheduler job ID — ensures re-registration replaces, not duplicates.
+JOB_ID: str = "session_cleanup"
+
+
+async def _run_cleanup_with_resources(settings: Settings) -> None:
+    """Delete all expired sessions from the database.
+
+    Args:
+        settings: The resolved application settings used for database access.
+    """
+    now_iso = utc_now().isoformat()
+    async with task_db(settings) as db:
+        deleted_count = await session_repo.delete_expired_sessions(db, now_iso)
+
+    log.info("session_cleanup_ran", deleted_count=deleted_count, cutoff_time=now_iso)
+
+
+async def _run_cleanup(app: FastAPI) -> None:
+    await _run_cleanup_with_resources(get_effective_settings(app))
+
+
+def register(app: FastAPI) -> None:
+    """Add (or replace) the session cleanup job in the application scheduler.
+
+    Must be called after the scheduler has been started (i.e., inside the
+    lifespan handler, after ``scheduler.start()``).
+
+    Args:
+        app: The :class:`fastapi.FastAPI` application instance whose
+            ``app.state.scheduler`` will receive the job.
+    """
+    settings = get_effective_settings(app)
+    app.state.scheduler.add_job(
+        _run_cleanup_with_resources,
+        trigger="interval",
+        seconds=SESSION_CLEANUP_INTERVAL,
+        kwargs={"settings": settings},
+        id=JOB_ID,
+        replace_existing=True,
+    )
+    log.info("session_cleanup_scheduled", interval_seconds=SESSION_CLEANUP_INTERVAL)