fix(obs): wire Prometheus endpoint for Spring Boot 4.0

Four Spring Boot 4.0-specific issues prevented /actuator/prometheus from working:

1. spring-boot-starter-micrometer-metrics missing — Spring Boot 4.0 splits
   Micrometer metrics export (including the Prometheus scrape endpoint) out of
   spring-boot-starter-actuator into its own starter. Added dependency.

2. management.prometheus.metrics.export.enabled not set — Spring Boot 4.0
   defaults metrics export to false (opt-in). Added the property to
   application.yaml.

3. SecurityConfig did not permit /actuator/prometheus — Spring Boot 4.0
   with Jetty serves the management port (8081) via the same security filter
   chain as the main port (8080). The previous commit's exclusion of
   ManagementWebSecurityAutoConfiguration was a no-op (that class no longer
   exists in Spring Boot 4.0); removed it and added the correct permitAll()
   rule. Updated the architecture comment in application.yaml to reflect the
   true filter-chain behaviour.

4. Reverted invalid FamilienarchivApplication.java change from the prior
   commit (ManagementWebSecurityAutoConfiguration import compiled against a
   class that does not exist in the Spring Boot 4.0 BOM).

Also adds ActuatorPrometheusIT — an integration test that asserts the
/actuator/prometheus endpoint returns 200 with jvm_memory_used_bytes without
credentials, serving as regression protection against future Spring Boot
upgrades silently breaking metrics collection.

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

backend/pom.xml

@@ -48,6 +48,11 @@
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-actuator</artifactId>
 		</dependency>
+		<!-- Spring Boot 4.0 splits Micrometer metrics export (incl. Prometheus scrape endpoint) into its own starter -->
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-starter-micrometer-metrics</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-validation</artifactId>