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/app/routes/files.py

@@ -339,10 +339,6 @@ def _serialize_folder(folder: FileFolder, count: int, size: int) -> dict:
     }
 
 
-def _own_folder_or_404(db: Session, user: User, folder_id: int) -> FileFolder:
-    return owned_or_404(db, FileFolder, folder_id, user, what="folder")
-
-
 @router.get("/folders")
 def list_folders(user: User = Depends(current_user), db: Session = Depends(get_db)) -> list[dict]:
     """The caller's folders with how many files each holds and their size. A source folder its
@@ -407,7 +403,7 @@ def update_folder(
 ) -> dict:
     """Rename one of the caller's own folders (a source folder's name is its origin's), and/or set
     how long any of their folders keeps a file (`folders.set_retention`)."""
-    folder = _own_folder_or_404(db, user, folder_id)
+    folder = owned_or_404(db, FileFolder, folder_id, user, what="folder")
     sent = body.model_fields_set
     if "name" in sent:
         if folder.kind != folders.FolderKind.OWN:
@@ -457,7 +453,7 @@ def delete_folder(
     folder_id: int, user: User = Depends(current_user), db: Session = Depends(get_db)
 ) -> dict:
     """Delete one of the caller's own folders; its files go back to their source folders."""
-    folder = _own_folder_or_404(db, user, folder_id)
+    folder = owned_or_404(db, FileFolder, folder_id, user, what="folder")
     try:
         moved = folders.delete_own(db, folder)
     except folders.FolderError as e: