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/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