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.

backend/app/main.py

@@ -28,6 +28,7 @@ log = logs.get_logger(__name__)
 from app import bodylimit, collation, logsink, state, sysconfig  # noqa: E402 — logging is configured first
 from app.config import settings  # noqa: E402 — logging is configured first
 from app.db import SessionLocal, engine  # noqa: E402 — logging is configured first
+from app.downloads import storage  # noqa: E402 — logging is configured first
 from app.plex import stream as plex_stream  # noqa: E402 — logging is configured first
 from app.plex import subtitles as plex_subtitles  # noqa: E402 — logging is configured first
 from app.routes import (  # noqa: E402 — logging is configured first
@@ -64,6 +65,10 @@ from app.scheduler import shutdown_scheduler, start_scheduler  # noqa: E402 —
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     log.info("Siftlode starting up")
+    problem = storage.files_root_problem(settings.files_root)
+    if problem:
+        # The rest of the app still serves; the files library cannot (the worker refuses to start).
+        log.critical("The files library is not reachable: %s. See docs/self-hosting.md.", problem)
     # The stored copy of the log (log_events). The entrypoint has already migrated the schema.
     logsink.start()
     # First-run: if the instance isn't configured yet, mint a fresh one-time setup token and print