refactor: merge gateway functionality into frontend container

Eliminated duplicate nginx containers by merging gateway reverse proxy
functionality into the frontend container. This simplifies deployment
and reduces resource usage.

Architecture changes:
- Frontend now serves both static files AND reverse proxies to backend
- Single nginx container handles all HTTP routing
- Gateway container removed from builds and Helm chart

Dockerfile.frontend changes:
- Added upstream backend configuration
- Added proxy locations for /api, /health, /metrics, /npm, /pypi, /go, /ws
- Added rate limiting for API and downloads
- Added WebSocket support
- Configurable via BACKEND_HOST and BACKEND_PORT env vars

Helm chart changes:
- Updated frontend deployment to configure backend connection
- Simplified ingress to single route (all traffic → frontend)
- Frontend proxies backend requests internally
- Removed separate frontend/api ingress configurations

GoReleaser changes:
- Removed gohoarder-gateway Docker build
- Now builds: server, scanner, migrate, frontend (4 images)

Benefits:
- Fewer containers to manage
- Reduced complexity in Docker Compose and Kubernetes
- Single point of configuration for routing
- Better resource utilization

.goreleaser.yaml

@@ -209,28 +209,7 @@ dockers_v2:
       - internal
       - config.yaml.example
 
-  # 4. Gateway - Nginx reverse proxy for unified deployment
-  - id: gohoarder-gateway
-    images:
-      - ghcr.io/lukaszraczylo/gohoarder-gateway
-    tags:
-      - "{{ .Version }}"
-      - latest
-    platforms:
-      - linux/amd64
-      - linux/arm64
-    dockerfile: Dockerfile.gateway
-    flags:
-      - "--pull"
-      - "--label=org.opencontainers.image.title=GoHoarder Gateway"
-      - "--label=org.opencontainers.image.description=Nginx reverse proxy for unified GoHoarder deployment"
-      - "--label=org.opencontainers.image.url=https://github.com/lukaszraczylo/gohoarder"
-      - "--label=org.opencontainers.image.source=https://github.com/lukaszraczylo/gohoarder"
-      - "--label=org.opencontainers.image.version={{ .Version }}"
-      - "--label=org.opencontainers.image.created={{ .Date }}"
-      - "--label=org.opencontainers.image.revision={{ .FullCommit }}"
-
-  # 5. Migration Engine - Database migration tool
+  # 4. Migration Engine - Database migration tool
   - id: gohoarder-migrate
     images:
       - ghcr.io/lukaszraczylo/gohoarder-migrate