Add the seam for immediate web-triggered summarization. Processor is the
optional single-video summarize port (nil = queue-only, unchanged behaviour);
ProcessingSet is an ephemeral, concurrency-safe set of in-flight (user,video)
ids so a status endpoint can show progress until the summary lands. State is
deliberately in-memory only — the DB holds the durable truth across restarts.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
