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.

.env.example

@@ -76,10 +76,10 @@ SCHEDULER_ENABLED=true
 # --- Download center ---
 # The Download Center adds a `worker` container (runs the yt-dlp/ffmpeg job loop) and a small
 # `bgutil-pot` sidecar (mints YouTube tokens) — both come up automatically with docker compose.
-# Downloaded media defaults to a Docker-managed named volume. Set DOWNLOAD_HOST_PATH to a host
+# Downloaded media defaults to a Docker-managed named volume. Set FILES_HOST_PATH to a host
 # directory instead — e.g. one your Plex server can read — to keep the Plex-style tree there.
 # The path must be writable by the container user (uid of `appuser`, 1000): chown 1000:1000 <dir>.
-# DOWNLOAD_HOST_PATH=/mnt/media/youtube
+# FILES_HOST_PATH=/mnt/media/youtube
 
 # --- Plex integration (optional) ---
 # The optional Plex module browses/plays your Plex library, playing the LOCAL physical file
@@ -89,6 +89,6 @@ SCHEDULER_ENABLED=true
 # to /plex-media inside the container (the local side of the admin "path map", whose default is
 # /data=/plex-media). Leave unset if you don't use Plex.
 # PLEX_MEDIA_HOST_PATH=/mnt/media
-# Optional: scratch dir for on-the-fly HLS remux segments. Defaults to DOWNLOAD_ROOT; point it at
+# Optional: scratch dir for on-the-fly HLS remux segments. Defaults to FILES_ROOT; point it at
 # fast local storage if your download volume is slow.
 # PLEX_HLS_DIR=/var/tmp/plex-hls