Initialize Darvest API Gateway BFF service

Set up Spring Cloud Gateway as the single ingress point for all Darvest frontend clients.

Includes:

• Route definitions for 12 downstream microservices under /api/* prefixes
• Resilience4j circuit breakers per route (stricter thresholds on financial routes: wallet, investment, distributions)
• Global CORS configuration via CorsWebFilter
• /fallback endpoint returning 503 with circuit breaker error details
• @RefreshScope on GatewayConfig for live config reload from Config Server
• GitLab CI pipeline (validate → test → build-publish → versioning)
• Dockerfile and Maven wrapper for containerized builds