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.

install.ps1

@@ -31,7 +31,7 @@ OAUTH_REDIRECT_URL=$url/auth/callback
 
 # Optional: store Download Center media in a host folder (e.g. one your Plex server reads) instead
 # of a Docker volume. Must be writable by uid 1000 (chown -R 1000:1000 <dir>).
-# DOWNLOAD_HOST_PATH=/mnt/media/youtube
+# FILES_HOST_PATH=/mnt/media/youtube
 "@ | Set-Content -Path .env -Encoding ascii
   Write-Host "Wrote .env (secrets generated)."
 }