Cron News Sentiment Backfill

One bounded, resumable slice of the 365-day news-sentiment backfill (NEWS-02). Auth: X-Cron-Secret.

Invoke repeatedly. Each call derives every symbol's resume point from news_sentiment_coverage / news_sentiment_daily themselves — no cursor table that could disagree with the data — skips the symbols already current, and works the rest in ticker order until chunk_size symbols are done or max_minutes elapses. Both bounds exist because the screener batch and the overnight prefetch own the shared 26.67 req/s AlphaVantage bucket from ~02:00 to ~23:30 UTC; an unbounded run would not fail, it would make the 06:00 batch late. The returned run reports stopped_because — chunk_size, max_minutes, universe_exhausted or universe_exhausted_with_errors; only universe_exhausted, with symbols_remaining == 0, means the backfill is finished. The _with_errors form is terminal too (nothing is left to claim) but it is NOT completion: symbols_failed symbols were claimed and left without ground, so it is to be reported, not read as a clean stop.

Sizing (measured in plan 94-02): >=13 provider calls per symbol per year, ~3x that on a name whose every month saturates the 1,000-item cap, over ~3,300 tickers — so roughly 43k-129k calls across the whole campaign, not one long run. Duplicate-safe: a second invocation while one is in flight gets cron_runs.status='skipped' from _run_tracked's atomic claim.