feat(MP-27): backend consolidation, gateway routing & service dockerfiles

Summary
- Backend Services (Entries, Results, Scheduling) haben Dockerfiles.
- Docker Compose Orchestrierung steht (DB + Gateway + Services).
- Gateway Routing für `entries-service` implementiert (StripPrefix, Path Rewrites).
- Health-Checks und 409-Conflict-Demo Endpunkt verifiziert.

Verification
- `docker compose up --build` -> Success
- `curl http://localhost:8081/api/entries` -> 200 OK (routed through Gateway)

Ref: MP-27

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

@@ -101,6 +101,31 @@ spring:
               filters:
                 - StripPrefix=1
 
+            # ==============================================================
+            # --- Entries-Service-Integration (MP-27) ---
+            # ==============================================================
+            - id: entries-service-route
+              uri: lb://entries-service
+              predicates:
+                - Path=/api/entries/**
+              filters:
+                - StripPrefix=1
+
+            # Mappe das Root "/api/entries" explizit auf die Service-Root "/"
+            - id: entries-service-root
+              uri: lb://entries-service
+              predicates:
+                - Path=/api/entries
+              filters:
+                - SetPath=/
+
+            - id: entries-service-root-slash
+              uri: lb://entries-service
+              predicates:
+                - Path=/api/entries/
+              filters:
+                - SetPath=/
+
   # ==============================================================
   # --- Members-Service-Integration (optional) ---
   # ==============================================================