fix(kevin_bridge): persist signal to signals table before audit row

End-to-end Phase 2 verification surfaced a FK violation: the bridge
publishes a TradeSignal to the Redis stream and writes
kevin_signal_bridge_state with signal_id, but signal_id has a FK to
the signals table — which was never populated for Kevin-emitted
signals (only the news+sentiment path wrote there).

AuditWriter.persist_signal() inserts the TradeSignal into the
signals table idempotently (on_conflict_do_nothing on the UUID PK)
before the bridge publishes to Redis. Bridge calls it as a new step
right before the XADD, so:
  1. Signal row exists in signals table
  2. XADD to signals:generated
  3. Audit row with signal_id FK now resolves

Verified live: mention #84 (synthetic NVDA buy, conviction 0.85)
emitted a signal, trade-executor consumed and correctly rejected
with outside_market_hours (market was closed at the time).

services/kevin_signal_bridge/main.py

@@ -131,6 +131,11 @@ class KevinBridge:
             take_profit_pct=Decimal(str(self.config.kevin_take_profit_pct)),
         )
 
+        # Persist to signals table FIRST so downstream FK
+        # (kevin_signal_bridge_state.signal_id → signals.id) resolves.
+        # On retry the on_conflict_do_nothing makes this idempotent.
+        await self.audit_writer.persist_signal(signal)
+
         try:
             stream_id = await self.publisher.publish(signal)
         except Exception: