feat(extraction): Phase 5 observability + error handling (5.7-5.12)
- 5.7: Enhanced structured logging with userId, productId, cacheHit, tokenCount
- 5.8: Metrics module (counters + histograms) + /extract/metrics endpoint
- 5.9: Grafana dashboard config for extraction-service (Loki queries)
- 5.10: Error mapping — sidecar errors → proper HTTP status codes (408, 429, 502, 503)
- 5.11: Circuit breaker for Python sidecar (5 failures → 30s OPEN)
- 5.12: Graceful degradation — circuit open returns 503, cached results still served
- 46 TS tests passing
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