FastAPI hello → the real main.py

Familiar decorator, new superpower

@app.route("/") from Flask and FastAPI's @app.get("/") look almost identical — same decorator idea from module 16, same "a function handles a request" model from Flask. The difference is what happens to the function's parameters.

In Flask, everything from the request is a string you convert by hand (float(request.args.get(...)), remember module 19?). In FastAPI, you write a type hint on the parameter, and FastAPI converts AND validates it for you — using Pydantic under the hood.