fix(files): review round 1, the server — the tray's window, the GC's locks, a file's origin, the upgrade path

- The queue keeps a done job by when it became done (ready_at), not by its last write: renaming,
  keeping or moving an old file, or the GC's expiry warning, no longer brings it back as just finished.
- GC expiry and eviction re-read each file under the asset→job locks the user routes take, and commit
  one file at a time: a delete, a re-download or a keep made while the GC runs is honoured, never a
  second release that frees a file another user still holds.
- A file records the origin it was placed under (download_jobs.source_key, migration 0105): an edit
  whose parent expired still goes back to its folder, not to Uploads.
- A folder a file is being put in is held (FOR SHARE) until it lands; a delete of it waits and takes
  that file home too, instead of failing on the foreign key.
- The hash route reads at most two files at once (429 past that) and holds no connection meanwhile.
- 0104's backfill dates old files by their request date only: no notice prod wrote names its job.
- The old DOWNLOAD_ROOT still finds the files; a files root that is not a mounted volume stops the
  worker and is logged by the API; the volume-copy recipe looks the project-prefixed names up.
- Stale comments about the /downloads mount and the Downloads page.

docker-compose.yml

@@ -63,7 +63,7 @@ services:
     restart: unless-stopped
 
   # Download worker: same image, runs the yt-dlp / ffmpeg job loop instead of the API. Shares the
-  # DB (job queue) + the downloads mount with the API.
+  # DB (job queue) + the /files mount with the API.
   worker:
     build:
       context: .