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.

.gitignore

@@ -29,9 +29,9 @@ frontend/e2e/.auth/
 *.sql.gz
 backups/
 
-# Download center: locally downloaded media (the DOWNLOAD_ROOT bind mount at the repo root).
+# Download center: locally downloaded media (the FILES_ROOT bind mount at the repo root).
 # Anchored with a leading slash so it does NOT also ignore the backend/app/downloads package.
-/downloads/
+/files/
 
 # OS / editor
 .DS_Store