Implement authentication middleware using Supabase JWT

Add middleware that validates Supabase JWT tokens, extracts the user id, and attaches it to the request context. Reject requests without valid tokens and make sure all protected endpoints require authentication.

Look for existing middleware or request-handling code in the project, likely in a `middleware/` or `src/middleware/` directory. Research Supabase JWT validation libraries or SDK methods. The middleware should validate tokens, extract the user ID, and attach it to the request. Identify protected endpoints to ensure they use this middleware. Test by making authenticated and unauthenticated requests to verify rejection and proper context attachment.

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