feat: unify environment scripts with start.sh

- Add unified start.sh script with subcommands (all/backend/frontend)
- Add process management (--stop, --status)
- Remove separate start_backend.sh and start_frontend.sh
- Update setup_dev_env.sh with pre-flight checks and --cpu-only/--skip-db options
- Update .env.example to remove sensitive data and add DIFY translation config
- Add .pid/ to .gitignore for process management

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -44,6 +44,9 @@ env/
 .env.local
 .env.*.local
 
+# ===== Process ID Files =====
+.pid/
+
 # ===== Logs =====
 logs/
 *.log