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_files_api.py

@@ -498,6 +498,50 @@ def test_an_upload_to_a_folder_it_may_not_use_is_refused_before_its_body_is_read
     assert db.execute(select(DownloadJob.id)).all() == before
 
 
+def test_an_upload_holds_its_folder_only_once_its_file_is_about_to_be_in_it(
+    db, make_user, as_user, _root, upload_headers, monkeypatch
+):
+    # Held from the start, the folder waited out the probe and the poster (up to a minute) before a
+    # rename or a delete of it could go on; not held at all, a delete could take it from under the file.
+    alice = make_user()
+    own = _own(db, alice.id)
+    steps: list[object] = []
+    real = folders.own_folder
+
+    def own_folder(db_, user_id, folder_id, *, hold=True):
+        steps.append(("folder", hold))
+        return real(db_, user_id, folder_id, hold=hold)
+
+    def probe(_p):
+        steps.append("probe")
+
+    monkeypatch.setattr(folders, "own_folder", own_folder)
+    monkeypatch.setattr(uploadmod.probe, "probe_json", probe)
+    r = as_user(alice).post(
+        "/api/files/upload", content=b"x", headers=upload_headers("a.txt", folder=own.id)
+    )
+    assert r.status_code == 200, r.text
+    # The route's own look before the body, the ingest's look, the probe, then the hold.
+    assert steps[-3:] == [("folder", False), "probe", ("folder", True)]
+
+
+def test_a_file_moved_holds_the_folder_it_goes_to(db, make_user, as_user, monkeypatch):
+    alice = make_user()
+    job = _file(db, alice.id)
+    own = _own(db, alice.id)
+    holds: list[bool] = []
+    real = folders.own_folder
+
+    def own_folder(db_, user_id, folder_id, *, hold=True):
+        holds.append(hold)
+        return real(db_, user_id, folder_id, hold=hold)
+
+    monkeypatch.setattr(folders, "own_folder", own_folder)
+    r = as_user(alice).patch(f"/api/files/{job.id}", json={"folder_id": own.id})
+    assert r.status_code == 200, r.text
+    assert holds == [True]
+
+
 def test_a_folder_deleted_while_its_upload_streams_refuses_the_upload(
     db, make_user, as_user, _root, _no_probe, upload_headers, monkeypatch
 ):