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/config.py

@@ -1,7 +1,7 @@
 from pathlib import Path
 from typing import Literal, get_args, get_origin
 
-from pydantic import Field, ValidationInfo, field_validator, model_validator
+from pydantic import AliasChoices, Field, ValidationInfo, field_validator, model_validator
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 # Shipped placeholder; production must override it. Used by the startup guard below.
@@ -171,9 +171,13 @@ class Settings(BaseSettings):
     feed_default_hidden_live: str = "live,upcoming"
 
     # --- Download center (yt-dlp) ---
-    # Root directory (inside the container) for downloaded media — a Docker volume/bind mount.
+    # Root directory (inside the container) for the files library — a Docker volume/bind mount.
     # Env-only (a mount path can't live in the DB). The worker lays a Plex-style tree under it.
-    files_root: str = "/files"
+    # DOWNLOAD_ROOT is its old name: an install still on the old compose file (mounted at
+    # /downloads, DOWNLOAD_ROOT=/downloads) keeps finding its files after it pulls a new image.
+    files_root: str = Field(
+        default="/files", validation_alias=AliasChoices("FILES_ROOT", "DOWNLOAD_ROOT", "files_root")
+    )
     # Whether THIS process runs the download worker loop. On for the dedicated worker
     # container, off for the API (which keeps the scheduler). Mirrors scheduler_enabled.
     worker_enabled: bool = False