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)

backend/.gitignore

@@ -2,7 +2,8 @@
 node_modules/
 
 # Environment variables
-.env
+.env.development
+.env.production
 
 # Logs
 logs/