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/pom.xml

@@ -31,6 +31,10 @@
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-actuator</artifactId>
         </dependency>
+        <dependency>
+            <groupId>io.micrometer</groupId>
+            <artifactId>micrometer-registry-prometheus</artifactId>
+        </dependency>
         <dependency>
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-jdbc</artifactId>