fix(auth): implement refresh token rotation endpoint

[Daksh7785]

Summary

Implemented a missing token refresh endpoint to support refresh token rotation, reducing the risk of token replay attacks, and added a secure logout endpoint for proper session termination.

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Root Cause

The authentication flow issued long-lived refresh tokens (7 days) but did not expose any endpoints to utilize these tokens. As a result, the application lacked refresh token rotation, meaning an intercepted refresh token could remain valid for its entire lifetime.

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Solution

• Added a POST /auth/refresh endpoint that accepts the refresh_token HTTP-only cookie.
• Leveraged supabase.auth.refresh_session() to securely validate the token, issue a new session, and automatically handle the invalidation/rotation of the previous refresh token.
• Set the newly rotated access and refresh tokens securely via _set_session_cookies().
• Added a POST /auth/logout endpoint that signs the user out of the Supabase backend and clears the session cookies.

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Files Changed

• backend/main.py:
  • Added auth_refresh logic.
  • Added auth_logout logic.

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Testing

• Build passed
• Lint passed
• Manual code verification completed for safe session handling

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Impact

• Breaking changes: No
• Performance impact: Negligible
• Security impact: High (Positive) - Severely limits the window of opportunity if a refresh token is stolen, complying with OAuth 2.0 best practices.
• Compatibility: Fully backward compatible with the current /auth/login cookie flow.

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Checklist

• Issue solved
• Build passes
• Lint passes
• No unrelated changes
• Ready for review

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