Recover an upload whose Mux URL expired, or cancel its Mux session.

Before minting anything the server asks Mux about the OLD upload: if it actually completed (the client's final-chunk ack was lost), the existing asset is ADOPTED onto this row instead of being orphaned — a fresh URL here would strand a live, billed Mux asset whose webhook no longer matches any mapping (upload-sdk spec §6.3/§8.3).

The guard accepts "errored" rows with no asset in addition to "waiting_for_upload": the status-sync poller flips timed-out uploads to errored, and that is exactly the state a client returning after a long gap finds.