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/app/downloads/og.py

@@ -135,7 +135,7 @@ def _watch_tags(token: str, db: Session) -> str | None:
         if image_dims is None and image.endswith("poster.jpg") and asset.poster_path:
             # safe_abs_path returns None if the poster file is missing (gc'd) or escapes the root —
             # guard it, else _jpeg_size(None) would raise an uncaught AttributeError on this public page.
-            poster_abs = storage.safe_abs_path(settings.download_root, asset.poster_path)
+            poster_abs = storage.safe_abs_path(settings.files_root, asset.poster_path)
             image_dims = _jpeg_size(poster_abs) if poster_abs else None
         if image_dims:
             tags.append(_tag("og:image:width", str(image_dims[0])))