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.

frontend/src/i18n/locales/en/errors.json

@@ -237,6 +237,7 @@
     "file_folder_name_taken": "You already have a folder with that name",
     "file_folder_not_own": "Only a folder you made can be renamed or deleted",
     "file_folder_gone": "That folder no longer exists",
+    "file_hash_busy": "Too many files are being checked right now — try again in a moment",
     "file_folder_not_its_source": "A file can only go into one of your own folders or back to the folder it came from",
     "file_no_preview": "There is no preview for this file"
   }

frontend/src/i18n/locales/hu/errors.json

@@ -237,6 +237,7 @@
     "file_folder_name_taken": "Már van ilyen nevű mappád",
     "file_folder_not_own": "Csak a saját mappádat nevezheted át vagy törölheted",
     "file_folder_gone": "Ez a mappa már nem létezik",
+    "file_hash_busy": "Most túl sok fájl ellenőrzése fut — próbáld újra kicsit később",
     "file_folder_not_its_source": "Egy fájl csak a saját mappáidba vagy vissza az eredeti mappájába kerülhet",
     "file_no_preview": "Ehhez a fájlhoz nincs előnézet"
   }