docs: finalize and verify Zipkin integration in system hardening roadmap

Marked Zipkin integration tasks as completed and verified in archived roadmaps. Documented fixes for gateway propagation type (`w3c` → `b3`) and Zipkin endpoint configuration. Removed redundant dependencies in `build.gradle.kts` and updated related session logs.

Signed-off-by: Stefan Mogeritsch <stefan.mo.co@gmail.com>

backend/infrastructure/gateway/build.gradle.kts

@@ -32,7 +32,8 @@ dependencies {
   implementation(libs.spring.cloud.starter.circuitbreaker.reactor.resilience4j)
 
   implementation(libs.spring.data.valkey)
-  implementation(libs.micrometer.tracing.bridge.brave)
+  // micrometer-tracing-bridge-brave + zipkin-reporter-brave + zipkin-sender-okhttp3
+  // werden transitiv via :backend:infrastructure:monitoring:monitoringClient bereitgestellt.
 
   testImplementation(projects.platform.platformTesting)
 }

backend/infrastructure/gateway/src/main/resources/application.yaml

@@ -52,9 +52,15 @@ management:
         enabled: true # Aktiviert /actuator/health/liveness und /readiness
   tracing:
     sampling:
-      probability: 1.0
+      probability: ${TRACING_SAMPLING_PROBABILITY:1.0}
     propagation:
-      type: "w3c"
+      # B3 ist das native Format von Brave/Zipkin. W3C wäre für OpenTelemetry.
+      # Alle Services nutzen Brave (via monitoring-client), daher muss das Gateway ebenfalls B3 verwenden.
+      type: "b3"
+  zipkin:
+    tracing:
+      # Lokal: Zipkin auf Port 9411. In Docker via ENV MANAGEMENT_ZIPKIN_TRACING_ENDPOINT überschrieben.
+      endpoint: ${MANAGEMENT_ZIPKIN_TRACING_ENDPOINT:http://localhost:9411/api/v2/spans}
 
 # --- Custom Service URLs ---
 # Default: Localhost (für Entwicklung ohne Docker)