Migration 0017_cron_triggers.sql created idx_cron_triggers_due on
(last_fired_at) with a comment claiming it serves the scheduler. The
actual scheduler query has no last_fired_at predicate — it filters
purely on `t.enabled = TRUE FOR UPDATE OF d SKIP LOCKED`. The index
has been pure write amplification with no read payoff.
Migration 0037 drops it. Reversible by re-running 0017's CREATE INDEX
if the planner story ever changes.
AUDIT.md anchor: F-M-001.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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