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/models.py

@@ -877,7 +877,7 @@ class MediaAsset(Base, TimestampMixin):
     upload_date: Mapped[date | None] = mapped_column(Date)
     thumbnail_url: Mapped[str | None] = mapped_column(Text)
     # yt-dlp's canonical page URL for the source (webpage_url) — the clean "downloaded from"
-    # reference shown on the Downloads page. Null until the worker extracts it (older/queued rows
+    # reference shown in the Files library. Null until the worker extracts it (older/queued rows
     # fall back to a URL derived from source_kind+source_ref).
     source_webpage_url: Mapped[str | None] = mapped_column(Text)
     error: Mapped[str | None] = mapped_column(Text)
@@ -1036,6 +1036,11 @@ class DownloadJob(Base, TimestampMixin, UpdatedAtMixin):
     folder_id: Mapped[int | None] = mapped_column(
         ForeignKey("file_folders.id", ondelete="RESTRICT"), index=True
     )
+    # The origin the file came from (the source folder's key), recorded where it is placed: an edit
+    # traces its origin through its parent, and that parent may be deleted or expire later — the
+    # file must still know which source folder it may go back to. NULL only for a file placed before
+    # the column existed outside a source folder; `folders.source_key_for` works those out.
+    source_key: Mapped[str | None] = mapped_column(String(260))
     kept: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")
     expires_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), index=True)
     expiry_warned: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")