📝 Use FastAPI lifespan instead of startup event

[Michele0303]

This updates the FastAPI tutorial examples to use the modern
FastAPI lifespan API instead of the deprecated startup event.

The on_startup() function is intentionally kept as a plain helper
and reused by the lifespan context manager, so that existing tutorial
tests can continue to call it directly without relying on the ASGI
lifecycle.

No behavioral changes are introduced, and all tests pass.

[github-actions]

📝 Docs preview

Last commit 4072384 at: https://8d9b4f3c.sqlmodel.pages.dev

Modified Pages

• https://8d9b4f3c.sqlmodel.pages.dev/tutorial/fastapi/delete/ - (before)
• https://8d9b4f3c.sqlmodel.pages.dev/tutorial/fastapi/limit-and-offset/ - (before)
• https://8d9b4f3c.sqlmodel.pages.dev/tutorial/fastapi/multiple-models/ - (before)
• https://8d9b4f3c.sqlmodel.pages.dev/tutorial/fastapi/read-one/ - (before)
• https://8d9b4f3c.sqlmodel.pages.dev/tutorial/fastapi/relationships/ - (before)
• https://8d9b4f3c.sqlmodel.pages.dev/tutorial/fastapi/response-model/ - (before)
• https://8d9b4f3c.sqlmodel.pages.dev/tutorial/fastapi/session-with-dependency/ - (before)
• https://8d9b4f3c.sqlmodel.pages.dev/tutorial/fastapi/simple-hero-api/ - (before)
• https://8d9b4f3c.sqlmodel.pages.dev/tutorial/fastapi/update-extra-data/ - (before)
• https://8d9b4f3c.sqlmodel.pages.dev/tutorial/fastapi/update/ - (before)

[rasifabdulrazak]

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