refactor: use environment-specific .env files

- Rename .env.example to .env.development.example
- Update docker-compose.yml to use env_file for dev backend
- Update .gitignore to ignore .env.development and .env.production
- Remove generic .env and .env.example in favor of:
  * .env.development (dev) + .env.development.example (template)
  * .env.production (prod) + .env.production.example (template)

docker-compose.yml

@@ -94,6 +94,8 @@ services:
       context: ./backend
       dockerfile: Dockerfile
     container_name: spotlightcam-backend
+    env_file:
+      - ./backend/.env.development
     expose:
       - "3000"
     volumes: