Backfill Earnings Calendar Endpoint

One-time deep backfill: pull AV EARNINGS history for every v79 ticker (~879) and insert all historical (symbol, report_date) rows into earnings_dates. Idempotent — ON CONFLICT DO NOTHING skips existing rows. Takes ~60-90 sec at AV's 1600/min rate.

After this completes, the earnings_iv_history backfill (run separately) can extract pre/post IV from iv_daily_snapshot's 18-year archive for every ticker × quarter → tens of thousands of crush observations instead of the ~500 we have today.