feat(01-02): create Docker Compose dev environment with hot reload

- docker-compose.yml orchestrates backend service with Air
- Bind mount for live code editing with cached consistency
- Named volume for persistent /data storage
- .air.toml configures Go hot reload on file changes
- Targets builder stage from multi-stage Dockerfile

docker-compose.yml

@@ -0,0 +1,18 @@
+services:
+  backend:
+    build:
+      context: .
+      dockerfile: docker/Dockerfile
+      target: builder  # Stop at build stage for dev
+    command: air -c .air.toml
+    ports:
+      - "32768:32768"
+    volumes:
+      - .:/workspace:cached  # Bind mount for live editing
+      - data:/data           # Named volume for persistent data
+    working_dir: /workspace
+    environment:
+      - CGO_ENABLED=0
+
+volumes:
+  data: