fix: zero compiler warnings + readiness endpoint

- Fix all 106 warnings: unused imports, dead struct fields (_prefix),
  dead_code annotations on bin-only items in ingest_worker.rs
- Add GET /ready endpoint (readiness probe, checks DB)
- Keep GET /health lightweight (liveness, no DB check)
- Add ServiceMonitor for Prometheus scraping
- Add tracing-subscriber json feature for structured logs
- 760 tests passing, 0 warnings, 0 errors

k8s/app/service-monitor.yaml

@@ -0,0 +1,15 @@
+apiVersion: monitoring.coreos.com/v1
+kind: ServiceMonitor
+metadata:
+  name: poimen-memory
+  namespace: poimen
+  labels:
+    app.kubernetes.io/name: poimen-memory
+spec:
+  selector:
+    matchLabels:
+      app.kubernetes.io/name: poimen-memory
+  endpoints:
+  - port: http
+    path: /metrics
+    interval: 30s