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).
