fix(manager-core): F-M-001 drop unused idx_cron_triggers_due

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>

crates/manager-core/migrations/0037_drop_idx_cron_triggers_due.sql

@@ -0,0 +1,12 @@
+-- F-M-001 (audit 2026-06-07): drop idx_cron_triggers_due.
+--
+-- Created in 0017_cron_triggers.sql with a comment claiming it serves
+-- the scheduler. The actual scheduler query is
+-- `... WHERE t.enabled = TRUE FOR UPDATE OF d SKIP LOCKED` — no
+-- `last_fired_at` predicate. The index is unused; the join is
+-- effectively a full table scan of cron details against enabled
+-- triggers (small N today, so the overhead is invisible — but the
+-- index is also pure write amplification with zero read payoff).
+--
+-- Drop is reversible by re-running the CREATE INDEX from 0017.
+DROP INDEX IF EXISTS idx_cron_triggers_due;