- Implement Task A7: background worker consuming signals:aider:events stream
- Parse AiderEventIn from Redis XREADGROUP messages
- Call IncidentEngine.process_signal for incident-worthy events
- Persist aider_events to PostgreSQL with optional incident_id FK
- XACK on success, preserve in pending list on DB failure (retry)
- ACK on parse failure (bad JSON avoids pending list jam)
- Match signal_worker.py pattern: no Active Sweeper (MVP)
- Unit tests: 4 tests covering incident creation, non-incident events, malformed payloads, engine failures
Tests: 37 passed (4 new + 33 existing regression)
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