Resolve ruststreamer shutdown and manual processing races

Address review findings in ruststreamer persistent processing. Acceptance criteria: - Every failed stream shutdown resolves the stopping reservation without leaving the stream name permanently blocked. - Failed shutdowns leave persisted processing jobs in an appropriate recoverable or terminal state. - Manual processing cannot discover or enqueue a recording session while a concurrent start is creating it. - Regression tests cover failed shutdown recovery/replacement and concurrent start versus manual transcode. - Rust formatting, checks, and relevant tests pass.