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/tests/test_upload_conversion.py

@@ -29,7 +29,7 @@ from sqlalchemy.orm import Session
 
 @pytest.fixture
 def _root(tmp_path, monkeypatch):
-    monkeypatch.setattr(settings, "download_root", str(tmp_path))
+    monkeypatch.setattr(settings, "files_root", str(tmp_path))
     return tmp_path