Integrate billing-service microservice: add API gateway routing, service discovery with Consul, Docker support, and Spring configuration. Update frontend with API integration, BillingRepository, and BillingViewModel.

frontend/core/network/src/commonMain/kotlin/at/mocode/frontend/core/network/ApiRoutes.kt

@@ -41,4 +41,10 @@ object ApiRoutes {
     const val ROOT = "/api/v1/series"
     fun stand(serieId: String) = "$ROOT/$serieId/stand"
   }
+
+  object Billing {
+    const val ROOT = "/api/v1/billing"
+    const val KONTEN = "$ROOT/konten"
+    fun buchungen(kontoId: String) = "$KONTEN/$kontoId/buchungen"
+  }
 }