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/config.py

@@ -173,7 +173,7 @@ class Settings(BaseSettings):
     # --- Download center (yt-dlp) ---
     # Root directory (inside the container) for downloaded media — a Docker volume/bind mount.
     # Env-only (a mount path can't live in the DB). The worker lays a Plex-style tree under it.
-    download_root: str = "/downloads"
+    files_root: str = "/files"
     # Whether THIS process runs the download worker loop. On for the dedicated worker
     # container, off for the API (which keeps the scheduler). Mirrors scheduler_enabled.
     worker_enabled: bool = False
@@ -345,8 +345,8 @@ class Settings(BaseSettings):
     plex_transcode_crf: int = 20
     plex_transcode_max_height: int = 1080
     # Where on-the-fly HLS remux segments are written (transient, reaped). Empty → a `.plex-hls`
-    # dir under download_root. On localdev the download_root is a slow Windows bind-mount, so point
-    # this at a fast container-local path (e.g. /var/tmp/plex-hls); prod's download_root is fast
+    # dir under files_root. On localdev the files_root is a slow Windows bind-mount, so point
+    # this at a fast container-local path (e.g. /var/tmp/plex-hls); prod's files_root is fast
     # local disk, so the default is fine there.
     plex_hls_dir: str = ""