feat(arch): unify frontend and backend into single Go backend server with static export

- Configure Next.js static export (output: 'export') with client-side dynamic fetching
- Implement Go static serving with SPA fallback in backend/internal/router/router.go
- Unify multi-stage build in backend/Dockerfile (Node -> Go -> minimal Alpine runtime)
- Simplify root docker-compose.yml to single anl-app service on port 8080
- Update REQUIREMENTS.md DM-03 and AC-17 normative contracts to v3.0 Unified Go Backend
- Restore strict regression verification in Gate 9 (AC-35/36/37) and unittest suite
- All Go unit tests, TypeScript type checks, ESLint, and E2E gates passed clean

.env.example

@@ -1,15 +1,16 @@
 # ==============================================================================
-# AI Agent Networking Lab (ANL) Unified Docker Compose Environment Configuration
+# AI Agent Networking Lab (ANL) Unified Single Backend Environment Configuration
 # ==============================================================================
 
 # REQUIRED: Secret Bearer token for mutating REST APIs and /console admin operations
 # Generate a secure random token (e.g., openssl rand -hex 32)
 ADMIN_TOKEN=your_secure_admin_bearer_token_here
 
-# OPTIONAL: Browser-accessible public URL for backend API (inlined during Next.js build)
-# Default: http://localhost:8080/api/v1
-NEXT_PUBLIC_API_BASE_URL=http://localhost:8080/api/v1
+# OPTIONAL: Public API base URL for client-side fetches.
+# In standard deployment (Go backend serving both static site and API on the same origin),
+# leave this UNSET or empty to use origin-relative '/api/v1' automatically.
+# Only specify an absolute URL (e.g., https://api.domain.com/api/v1) for cross-origin setups.
+NEXT_PUBLIC_API_BASE_URL=
 
-# OPTIONAL: Host port mappings
-BACKEND_PORT=8080
-FRONTEND_PORT=3000
+# OPTIONAL: Host port mapping for unified container (default: 8080)
+APP_PORT=8080