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_failure_lines.py

@@ -258,7 +258,7 @@ def test_an_edit_that_crashes_is_said_once(sink, finalized, monkeypatch, tmp_pat
     """The line `_process_edit`'s handler used to log before `_fail_edit` logged it again."""
     (tmp_path / "src.mp4").write_bytes(b"x")
     monkeypatch.setattr(pipeline, "SessionLocal", _EditDb)
-    monkeypatch.setattr(pipeline.settings, "download_root", str(tmp_path))
+    monkeypatch.setattr(pipeline.settings, "files_root", str(tmp_path))
     monkeypatch.setattr(pipeline, "_edit_staging_dir", lambda asset_id: tmp_path / "staging")
 
     def odd_spec(*a, **k):
@@ -300,7 +300,7 @@ def test_a_failed_download_line_does_not_put_back_what_the_adapter_took_out(sink
         RuntimeError("dictionary changed size during iteration"),
         RecursionError("maximum recursion depth exceeded"),
         NotImplementedError(),
-        FileNotFoundError(2, "No such file", "/downloads/Some_Channel/Season_2024/A_Private_Title"),
+        FileNotFoundError(2, "No such file", "/files/Some_Channel/Season_2024/A_Private_Title"),
     ],
     ids=lambda e: type(e).__name__,
 )
@@ -499,11 +499,9 @@ def test_a_failed_poster_or_storyboard_names_no_file(sink, tmp_path):
 @pytest.mark.parametrize(
     "err",
     [
-        subprocess.TimeoutExpired(
-            ["ffmpeg", "-i", "/downloads/Some_Channel/A_Private_Title.mp4"], 30
-        ),
-        FileNotFoundError(2, "No such file", "/downloads/Some_Channel/A_Private_Title.jpg"),
-        subprocess.CalledProcessError(1, ["ffmpeg", "-i", "/downloads/Some_Channel/x.mp4"]),
+        subprocess.TimeoutExpired(["ffmpeg", "-i", "/files/Some_Channel/A_Private_Title.mp4"], 30),
+        FileNotFoundError(2, "No such file", "/files/Some_Channel/A_Private_Title.jpg"),
+        subprocess.CalledProcessError(1, ["ffmpeg", "-i", "/files/Some_Channel/x.mp4"]),
     ],
     ids=lambda e: type(e).__name__,
 )
@@ -523,9 +521,7 @@ def test_a_thumbnail_that_cannot_be_copied_names_no_file(sink, monkeypatch, tmp_
     src.write_bytes(b"\xff\xd8\xff rest")
 
     def refuse(a, b):
-        raise PermissionError(
-            13, "Permission denied", "/downloads/Some_Channel/A_Private_Title.jpg"
-        )
+        raise PermissionError(13, "Permission denied", "/files/Some_Channel/A_Private_Title.jpg")
 
     monkeypatch.setattr(editmod.shutil, "copyfile", refuse)
     assert editmod.thumbnail_to_jpg(src, tmp_path / "out.jpg") is None