devops(backend): expose Prometheus metrics endpoint + OTLP trace export from Spring Boot

- Add micrometer-registry-prometheus (BOM-managed) to expose /actuator/prometheus
- Add micrometer-tracing-bridge-otel (BOM-managed) for Micrometer → OTel tracing bridge
- Add opentelemetry-spring-boot-starter 2.27.0 (pinned — not in Spring Boot BOM)
- Move management to port 8081 so Prometheus scrapes directly inside archiv-net,
  bypassing both Caddy and Spring Security's session-authenticated filter chain
- Configure otel.service.name and OTLP endpoint (default localhost:4317 for CI safety)
- Set tracing sampling probability to 1.0 in base config; override via env var in compose
- Add OTEL_EXPORTER_OTLP_ENDPOINT + MANAGEMENT_TRACING_SAMPLING_PROBABILITY to docker-compose.yml
- Expose management port 8081 inside archiv-net for Prometheus scraping
- Disable trace export in application-test.yaml (probability: 0.0) for deterministic CI

OTLP export failures are non-fatal; app starts cleanly without Tempo running.
Closes #576

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/application.yaml

@@ -45,9 +45,34 @@ server:
   forward-headers-strategy: native
 
 management:
+  server:
+    # Management port is separate from the app port so that:
+    # (a) Caddy never proxies /actuator/* (it only routes :8080 → the app port)
+    # (b) Prometheus scrapes backend:8081 directly inside archiv-net, not via Caddy
+    # (c) Spring Security's session-authenticated filter chain on :8080 never sees actuator requests
+    port: 8081
+  endpoints:
+    web:
+      exposure:
+        include: health,info,prometheus,metrics
+  endpoint:
+    prometheus:
+      enabled: true
   health:
     mail:
       enabled: false
+  tracing:
+    sampling:
+      probability: 1.0   # 100% in dev; override via MANAGEMENT_TRACING_SAMPLING_PROBABILITY in prod compose
+
+# OpenTelemetry trace export — failures are non-fatal (app starts cleanly without Tempo running)
+# The default http://localhost:4317 ensures CI compatibility when no observability stack is present.
+otel:
+  service:
+    name: familienarchiv-backend
+  exporter:
+    otlp:
+      endpoint: ${OTEL_EXPORTER_OTLP_ENDPOINT:http://localhost:4317}
 
 springdoc:
   api-docs:

backend/src/test/resources/application-test.yaml

@@ -13,3 +13,10 @@ spring:
     password: test
   mail:
     host: localhost
+
+# Disable trace export in tests — prevents OTLP connection attempts when no Tempo is running.
+# Sampling probability 0.0 means no spans are created, so no export is attempted.
+management:
+  tracing:
+    sampling:
+      probability: 0.0