add metrics ingestion diagnostics and upgrade cameleer3-common to 0.0.3

- Add logging to MetricsController: warn on parse failures, debug on
  received metrics, buffer depth on 503
- Add GET /api/v1/admin/database/metrics-pipeline diagnostic endpoint
  (buffer depth, row count, distinct agents/metrics, latest timestamp)
- Fix BackpressureIT test JSON to match actual MetricsSnapshot schema
  (collectedAt/metricName/metricValue instead of timestamp/metrics)
- Upgrade cameleer3-common from 1.0-SNAPSHOT to 0.0.3 (adds engineLevel)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pom.xml

@@ -28,7 +28,7 @@
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <java.version>17</java.version>
         <jackson.version>2.17.3</jackson.version>
-        <cameleer3-common.version>1.0-SNAPSHOT</cameleer3-common.version>
+        <cameleer3-common.version>0.0.3</cameleer3-common.version>
         <testcontainers.version>2.0.3</testcontainers.version>
     </properties>