test(integration): add e2e testing suite and infrastructure

Introduces a comprehensive integration testing setup and local development environment.

Changes include:
- Add `docker-compose.yml` for local stack orchestration.
- Add `docker-compose.integration.yml` for the integration test environment.
- Create `mock-upstream` service to simulate external LLM provider responses.
- Implement Go-based end-to-end tests verifying control plane configuration and data plane routing.
- Add `integration_test.sh` for quick connectivity verification.

docker-compose.integration.yml

@@ -0,0 +1,80 @@
+version: '3.8'
+
+services:
+  postgres:
+    image: postgres:15-alpine
+    environment:
+      POSTGRES_USER: postgres
+      POSTGRES_PASSWORD: postgres
+      POSTGRES_DB: ezapi
+    ports:
+      - "5434:5432"
+    volumes:
+      - pgdata-int:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U postgres"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+
+  redis:
+    image: redis:7-alpine
+    ports:
+      - "6380:6379"
+    volumes:
+      - redisdata-int:/data
+    healthcheck:
+      test: ["CMD", "redis-cli", "ping"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+
+  mock-upstream:
+    build:
+      context: ./integration/mock-upstream
+      dockerfile: Dockerfile
+    ports:
+      - "8082:8082"
+    healthcheck:
+      test: ["CMD", "wget", "-qO-", "http://localhost:8082/health"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+
+  ez-api:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    environment:
+      EZ_API_PORT: 8080
+      EZ_PG_DSN: host=postgres user=postgres password=postgres dbname=ezapi port=5432 sslmode=disable
+      EZ_REDIS_ADDR: redis:6379
+    ports:
+      - "8080:8080"
+    depends_on:
+      postgres:
+        condition: service_healthy
+      redis:
+        condition: service_healthy
+
+  balancer:
+    build:
+      context: ../balancer
+      dockerfile: Dockerfile
+    environment:
+      EZ_BALANCER_PORT: 8081
+      EZ_REDIS_ADDR: redis:6379
+      EZ_BALANCER_REFRESH_SECONDS: 2
+    ports:
+      - "8081:8081"
+    depends_on:
+      redis:
+        condition: service_healthy
+      ez-api:
+        condition: service_started
+      mock-upstream:
+        condition: service_healthy
+
+volumes:
+  pgdata-int:
+  redisdata-int: