fix(files): review round 2, the server — locks the GC and folders read under, the mount check, the old mount name

- GC: the expiry warning marks each file conditionally on the expiry it saw, one commit each (a file
  deleted meanwhile no longer fails the run; one re-timed keeps its fresh expiry unwarned); eviction
  counts the disk again when a candidate was freed meanwhile, so it never evicts for space already free.
- Folders: a retention change locks the folder and its files, and a finishing file reads the retention
  under a share lock, so neither misses the other; a folder delete locks the files it takes home (a
  file moved out meanwhile stays put, one deleted meanwhile no longer fails the delete).
- Upload: the folder is checked before the probe and held only once the job names it.
- Dedup enqueue locks the cached asset, so a release meanwhile yields a fresh one instead of a 500.
- Mount check reads the kernel's mount table instead of comparing devices (the vfs storage driver gave
  a mounted volume the device of /), and only inside a container; the API also logs a fresh, empty
  volume in the library's place.
- Compose files fall back to DOWNLOAD_HOST_PATH; the upgrade notes cover the from-source pull.
- Folder routes call owned_or_404 directly.

backend/tests/test_storage.py

@@ -177,15 +177,29 @@ def test_sidecars_keep_the_full_basename_when_the_title_has_a_dot(tmp_path: Path
     assert "Chan_-_2026-08-12_-_Sonka_Hazilag.nfo" not in names  # the old truncated orphan
 
 
+def _mounts(*points):
+    """A /proc/self/mountinfo table with these mount points (the fields around them are filler)."""
+    return "\n".join(f"{i} 1 0:{i} / {p} rw - ext4 /dev/x rw" for i, p in enumerate(points, 22))
+
+
 def test_a_files_root_on_the_containers_own_disk_is_refused(tmp_path):
     # The image's empty /files, left unmounted by an old compose file: every file would 404 and
     # every download would go with the container.
-    assert "not a mounted volume" in files_root_problem(
-        str(tmp_path), system_root=str(tmp_path.parent)
-    )
+    root = str(tmp_path)
+    plain = _mounts("/", "/proc", "/etc/hosts")
+    assert "not a mounted volume" in files_root_problem(root, plain, in_container=True)
     assert "does not exist" in files_root_problem(str(tmp_path / "gone"))
-    # On another filesystem than the container's own, and writable: the volume.
-    assert files_root_problem(str(tmp_path), system_root="/proc") is None
+    # Its own mount, or inside one: the volume — whatever device it reports. On a storage driver that
+    # keeps the container's root on the host's disk (vfs) a volume has the same device as /.
+    assert files_root_problem(root, _mounts("/", root), in_container=True) is None
+    assert files_root_problem(root, _mounts("/", str(tmp_path.parent)), in_container=True) is None
+    # A mount point with a space in it, as the kernel escapes it.
+    spaced = tmp_path / "my files"
+    spaced.mkdir()
+    escaped = str(spaced).replace(" ", "\\040")
+    assert files_root_problem(str(spaced), _mounts("/", escaped), in_container=True) is None
+    # Outside a container a plain directory is a fine root.
+    assert files_root_problem(root, plain, in_container=False) is None
 
 
 def test_the_old_name_of_the_files_root_still_finds_the_files(monkeypatch):