fix(retention): add cleanup sweeps + CalDAV orphan reconciliation

Drift-audit Group 4 (retention / unbounded growth):

- CalDAV pull now reconciles deletions: a previously-synced event whose
  caldav_uid no longer appears remotely within the synced window is
  soft-deleted (one batch_id per run, restorable), so a remote delete
  propagates locally instead of orphaning forever. Guarded on a non-empty
  fetch so a spurious empty result can't wipe every local copy. Also wrap
  the blocking fetch in a 120s wait_for and log run duration.
- Notifications: hourly loop now purges read notifications older than 30d
  (unread kept). Table no longer grows without bound.
- Auth tokens: new daily sweep deletes password-reset / invitation tokens
  whose validity window ended >7d ago; wired via start_auth_token_retention_loop
  in app startup. Both tables previously only flipped used=True, never pruned.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/fabledassistant/services/notifications.py

@@ -236,6 +236,28 @@ async def check_due_tasks() -> None:
                 logger.exception("Failed to send task reminder for user %d", user_id)
 
 
+# Read notifications are kept this long before the hourly sweep deletes them;
+# unread are kept regardless. Without a sweep the table grows without bound.
+_NOTIFICATION_RETENTION_DAYS = 30
+
+
+async def purge_old_read_notifications(retention_days: int = _NOTIFICATION_RETENTION_DAYS) -> int:
+    """Delete already-read in-app notifications older than retention_days."""
+    from datetime import timedelta
+    from sqlalchemy import delete
+    from fabledassistant.models.notification import Notification
+    cutoff = datetime.now(timezone.utc) - timedelta(days=retention_days)
+    async with async_session() as session:
+        result = await session.execute(
+            delete(Notification).where(
+                Notification.read_at.isnot(None),
+                Notification.read_at < cutoff,
+            )
+        )
+        await session.commit()
+        return result.rowcount or 0
+
+
 async def _notification_loop() -> None:
     while True:
         await asyncio.sleep(3600)  # hourly
@@ -243,6 +265,12 @@ async def _notification_loop() -> None:
             await check_due_tasks()
         except Exception:
             logger.exception("Error in notification loop")
+        try:
+            removed = await purge_old_read_notifications()
+            if removed:
+                logger.info("Notification retention: deleted %d read notification(s)", removed)
+        except Exception:
+            logger.exception("Error in notification retention cleanup")
 
 
 def start_notification_loop() -> None: