Create deployment plan for static frontend and FastAPI backend

[MyouzzZ]

Summary

• document deployment options for the static frontend and FastAPI backend
• cover backend hosting commands, frontend hosting choices, CORS, API URL configuration, and smoke tests
• document environment and secret requirements for GOOGLE_API_KEY, PORT, and future allowed origins

Closes #34.

Validation

• Reviewed README.md, SETUP_GUIDE.md, backend/main.py, and backend/requirements.txt
• Checked current frontend API base and backend environment variables
• git diff --check -- docs/deployment-plan.md