Add auth/authorization middleware

Add auth/authorization middleware to the FastAPI app, using the identity/provider.py seam. Currently a non-goal placeholder per NOTES.md; needs design + implementation.

The issue mentions using the identity/provider.py seam. Start by examining that file and the existing FastAPI app structure to understand the current authentication placeholder. Review NOTES.md for context on why this is a non-goal and what design is needed. Implementation will involve creating middleware that integrates with the provider seam to handle auth and authorization.

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