feat: add Micrometer Prometheus metrics to server

Adds micrometer-registry-prometheus and exposes /api/v1/prometheus
endpoint (unauthenticated for scraping). ServerMetrics component
provides business metrics beyond default JVM/HTTP:

Gauges: agents by state, SSE connections, buffer depths (execution,
processor, log, metrics), accumulator pending exchanges.

Counters: ingestion drops (buffer_full, no_agent, no_identity),
agent transitions (went_stale, went_dead, recovered), deployment
outcomes (running, failed, degraded), auth failures (invalid_token,
revoked, oidc_rejected).

Timers: ClickHouse flush duration by type, deployment duration.

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

cameleer3-server-app/src/main/resources/application.yml

@@ -100,7 +100,7 @@ management:
     web:
       base-path: /api/v1
       exposure:
-        include: health
+        include: health,prometheus
   endpoint:
     health:
       show-details: always