RunOnce walks the user's subscriptions, upserts each candidate video (assigning
its durable store id), skips videos already summarized via the store's
SeenVideoIDs (cross-restart dedup the engine's in-memory map can't provide),
and processes the rest through the engine. Loop adds an optional poll cadence;
per-item errors are collected, not fatal. Tested with fakes — no live deps.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
