feat(api): add /api/metrics/manifold-coverage by-category endpoint

Measure real Manifold coverage per semantic market category counted by UNIQUE
market (not audit rows, which are inflated by repeated re-evaluation). Base table
is manifold_match_audit filtered to v3_outcome_guard; each poly_market_id is
collapsed to one row, LEFT JOIN trades for family_key, category inferred from
family_key when present else from poly_question (gubernatorial / mayoral / senate
/ primary-republican / primary-democrat / big-tech / geopolitics / other).

Per category: unique_evaluated / unique_accepted / unique_rejected /
unique_no_results / coverage_rate, ordered by unique_evaluated DESC. Summary adds
total_unique_evaluated, total_unique_accepted, overall_coverage_rate,
categories_with_coverage. Read-only: new db method + endpoint only; matcher,
scheduler, cooldowns, thresholds and exposure untouched.

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

api/main.py

@@ -243,6 +243,32 @@ async def get_manifold_matches():
     return data
 
 
+@app.get("/api/metrics/manifold-coverage")
+async def get_manifold_coverage():
+    """Manifold coverage by semantic market category, counted by UNIQUE market.
+
+    Unlike the raw audit counters (which count per-attempt and are inflated by the
+    trading loop re-evaluating the same markets), this measures real coverage:
+    how many DISTINCT markets in each category the matcher found an accepted
+    Manifold counterpart for. Base table is manifold_match_audit filtered to the
+    current matcher (v3_outcome_guard); category is inferred from trade family_key
+    when available, otherwise from the question text.
+
+    coverage_by_category — one entry per category, ordered by unique_evaluated DESC:
+      category          — gubernatorial | mayoral | senate | primary-republican |
+                          primary-democrat | big-tech | geopolitics | other
+      unique_evaluated  — distinct markets audited in this category
+      unique_accepted   — distinct markets with at least one accepted match
+      unique_rejected   — distinct markets with at least one rejected match
+      unique_no_results — distinct markets with at least one no_results outcome
+      coverage_rate     — unique_accepted / unique_evaluated (null if evaluated=0)
+    summary — total_unique_evaluated, total_unique_accepted, overall_coverage_rate
+      (null if nothing evaluated), categories_with_coverage (categories with
+      unique_accepted > 0).
+    """
+    return await db.get_manifold_coverage_by_category()
+
+
 @app.get("/api/summary")
 async def get_summary():
     """Dashboard summary card data.