feat(files): every file sits in a library folder, and keeps its own expiry

The Files library starts here. A user's every download, clip and upload now
lands in a source folder made per origin (YouTube, Plex clips, uploads, one per
other site), decided once in downloads/folders.py at every place a job is made.
How long a file stays moves from the shared asset to the user's job: stamped
from its folder when it is done, never for a file the user keeps. The GC warns
each user once, then takes the expired file out of their library; the cache cap
never evicts a kept file, and an evicted one leaves every holder's library.

The media mount is /files now (FILES_ROOT, FILES_HOST_PATH, volume
siftlode_files); docs/self-hosting.md says how to carry an old volume over.
Migration 0102 places every existing job and carries each done file's expiry
over from its asset.

safe_abs_path takes a missing path and answers None, which drops one baselined
type error.

backend/alembic/versions/0050_asset_poster.py

@@ -6,7 +6,7 @@ Create Date: 2026-07-07
 
 Some sources (a direct media URL like a reddit v.redd.it HLS manifest) carry no thumbnail, so the
 download shows a blank image box. The worker now extracts a representative frame with ffmpeg and
-saves it as a poster sidecar; `poster_path` (relative to download_root) records it so the card /
+saves it as a poster sidecar; `poster_path` (relative to files_root) records it so the card /
 watch page / link preview can fall back to it. Nullable — only set when there was no thumbnail.
 """