chore(gradle): centralize repository config in settings.gradle.kts and simplify BOM management

### Summary
- Removed `repositories` blocks from individual `build.gradle.kts` files, moving configuration to `settings.gradle.kts`.
- Replaced custom Spring Boot constraints in the platform BOM with the Spring Boot BOM for cleaner dependency management.
- Explicitly added `webflux` dependency for Gateway to handle transitivity changes in Spring Boot 4.x.

backend/infrastructure/gateway/build.gradle.kts

@@ -24,6 +24,10 @@ dependencies {
   implementation(projects.backend.infrastructure.monitoring.monitoringClient)
 
   // === GATEWAY-SPEZIFISCHE ABHÄNGIGKEITEN ===
+  // KORREKTUR: Explizite Deklaration von WebFlux, da es in Spring Boot 4.x
+  // anscheinend nicht mehr vollständig transitiv vom Gateway-Starter eingebunden wird.
+  implementation(libs.spring.boot.starter.webflux)
+
   // Kern-Gateway inkl. Security, Actuator, CircuitBreaker, Discovery
   implementation(libs.bundles.gateway.core)
   // Ergänzende Observability (Logging, Jackson)