How do you add GraphQL support to a FastAPI application with Strawberry?

# pip install strawberry-graphql[fastapi]
import strawberry
from strawberry.fastapi import GraphQLRouter
from fastapi import FastAPI
from typing import Optional

# Define types with Strawberry (type hints = schema)
@strawberry.type
class User:
    id:       int
    username: str
    email:    str

fake_users = [
    User(id=1, username="alice", email="alice@example.com"),
    User(id=2, username="bob",   email="bob@example.com"),
]

# Query type
@strawberry.type
class Query:
    @strawberry.field
    def users(self) -> list[User]:
        return fake_users

    @strawberry.field
    def user(self, id: int) -> Optional[User]:
        return next((u for u in fake_users if u.id == id), None)

# Mutation type
@strawberry.type
class Mutation:
    @strawberry.mutation
    def create_user(self, username: str, email: str) -> User:
        new_user = User(id=len(fake_users)+1, username=username, email=email)
        fake_users.append(new_user)
        return new_user

schema = strawberry.Schema(query=Query, mutation=Mutation)
graphql_app = GraphQLRouter(schema)

app = FastAPI()
app.include_router(graphql_app, prefix="/graphql")

# REST routes coexist happily
@app.get("/health")
def health():
    return {"status": "ok"}