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_storage.py

@@ -100,7 +100,7 @@ class TestDownloadFilename:
 
 
 class TestSafeAbsPath:
-    """The path-traversal guard: only paths that resolve INSIDE download_root AND exist are returned."""
+    """The path-traversal guard: only paths that resolve INSIDE files_root AND exist are returned."""
 
     def test_returns_the_path_for_a_file_inside_the_root(self, tmp_path: Path):
         (tmp_path / "sub").mkdir()