fix(download): stop stale aria2 GIDs from spamming errors after queue clears

- Log expected "GID not found" tellStatus probes at DEBUG, and treat a
  forgotten GID as permanent so the poll loop recovers immediately
  instead of burning 4 retries x 3s of ERROR lines per cycle
- cancel_download tolerates a forgotten GID and always pops the
  in-memory transfer so concurrent polls cannot re-register a
  cancelled download
- Restore sweep deletes aria2 state records with no resolvable target
  path instead of skipping them forever
- Clearing the download queue now also cancels in-memory tasks, removes
  live aria2 transfers and drops persisted state for the cleared ids
  (partial files on disk are preserved)

py/services/download_coordinator.py

@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 import logging
-from typing import Any, Awaitable, Callable, Dict, Optional
+from typing import Any, Awaitable, Callable, Dict, Iterable, Optional
 
 from .downloader import DownloadProgress
 
@@ -186,6 +186,14 @@ class DownloadCoordinator:
         download_manager = await self._download_manager_factory()
         return await download_manager.get_active_downloads()
 
+    async def discard_cleared_downloads(self, download_ids: Iterable[str]) -> int:
+        """Tear down in-memory/aria2 tracking for queue-cleared downloads."""
+
+        if not download_ids:
+            return 0
+        download_manager = await self._download_manager_factory()
+        return await download_manager.discard_cleared_downloads(download_ids)
+
     def _parse_optional_int(self, value: Any, field: str) -> Optional[int]:
         """Parse an optional integer from user input."""