feat(manifold): add persistent cooldowns to reduce redundant evaluations

The trading loop re-evaluated the same ~22 politics/tech markets every ~60s,
flooding manifold_match_audit with ~76k rows (~3,500 attempts/market) of which
none carried new information, making the metrics uninterpretable.

Add per-market persistent cooldowns:
- New table manifold_eval_cooldown (poly_market_id PK, last_evaluated_at,
  last_status, retry_after, cooldown_reason) created via run_migrations.
- bayesian.evaluate() consults the cooldown BEFORE calling the matcher and skips
  the call entirely while now() < retry_after — no matcher call, no audit row,
  no signal (equivalent to no_results). After a real evaluation it upserts the
  cooldown with a verdict-dependent backoff: no_results/low_score/outcome_mismatch/
  ambiguous_inversion -> 24h, conditional_market -> 7d, accepted -> 1h.
- manifold.py stays a pure client/matcher with no DB access.
- New db methods get_manifold_cooldown / upsert_manifold_cooldown.
- /api/metrics/manifold-matches summary gains unique_markets
  {evaluated, accepted, coverage_rate} for per-market (not per-attempt) coverage.

Matching logic, MANIFOLD_MATCHER_VERSION, MANIFOLD_LOGODDS_WEIGHT, edge/exposure
thresholds and existing audit rows are unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

bot/data/schema.sql

@@ -291,3 +291,29 @@ CREATE TABLE IF NOT EXISTS checkpoint_alerts (
     fired_at        TIMESTAMPTZ NOT NULL,
     last_fired_at   TIMESTAMPTZ
 );
+
+-- ─────────────────────────────────────────────────────────────────────────────
+-- Manifold evaluation cooldown — per-market backoff for the Manifold matcher
+--
+-- The trading loop re-evaluates the same ~stable set of politics/tech markets
+-- every cycle (~60s).  Most resolve to a stable terminal verdict (no Manifold
+-- coverage, low-score, outcome mismatch, conditional market) that will not change
+-- on the next cycle.  Re-querying them every minute floods manifold_match_audit
+-- with redundant rows and makes the metrics uninterpretable.
+--
+-- This table records, per poly_market_id, when the market was last evaluated and
+-- the earliest time it should be evaluated again (retry_after).  evaluate() in
+-- bot/strategy/bayesian.py consults it BEFORE calling the matcher and skips the
+-- call (and the audit write) entirely while now() < retry_after.
+--
+-- last_status / cooldown_reason are stored for observability only.
+-- ─────────────────────────────────────────────────────────────────────────────
+CREATE TABLE IF NOT EXISTS manifold_eval_cooldown (
+    poly_market_id    TEXT PRIMARY KEY,
+    last_evaluated_at TIMESTAMPTZ NOT NULL,
+    last_status       TEXT NOT NULL,
+    retry_after       TIMESTAMPTZ NOT NULL,
+    cooldown_reason   TEXT
+);
+
+CREATE INDEX IF NOT EXISTS idx_mfld_cooldown_retry ON manifold_eval_cooldown(retry_after);