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/worker.py

@@ -715,7 +715,7 @@ def main() -> None:
     _target = max(1, settings.download_worker_concurrency)  # used only if the DB can't answer
     threads: dict[int, threading.Thread] = {}
     level = _sync_knobs(threads, None)
-    log.info("Download worker started (concurrency=%d, root=%s).", _target, settings.download_root)
+    log.info("Download worker started (concurrency=%d, root=%s).", _target, settings.files_root)
     while not _stop.wait(_KNOB_SECONDS):
         level = _sync_knobs(threads, level)
     log.info("Download worker stopping…")