[ENH] FastAPI Stack

[b-jonathan]

Is your feature request related to a problem?
I want to be able to make a FastAPI app

Describe the solution
FastAPI Stack

Description:
Add FastAPI as a new backend option alongside Express.

Requirements:

• Single-file project structure: main.py with routes inline
• Set up Python virtual environment using venv + requirements.txt
• Install dependencies: fastapi, uvicorn, python-dotenv
• Include CORS middleware configured with FRONTEND_ORIGIN from .env (same pattern as Express)
• Generate .env with PORT=4000 and FRONTEND_ORIGIN=http://localhost:3000
• Compatible with MongoDB as a database (add templates under mongodb/fastapi/)
• Add lint tooling: ruff for linting/formatting with check and fix scripts
• Add .gitignore entries for pycache/, venv/, .env*
• Add templates under fastapi/ in the embedded template directory
• Register as "fastapi" in the stack registry