feat(backend): Resolve backend issues 581, 822, 823, 580

[software321dev]

This PR introduces the following backend fixes and enhancements:

• Issue Enforce maximum length on displayName and bio at the handler level #581: Added maximum length validation for displayName (50 chars) and bio (500 chars) in the upsert profile handler to ensure correct VALIDATION_ERROR format.
• Issue JWT secret falls back to a hardcoded string when JWT_SECRET environment variable is missing #822: Removed the hardcoded fallback for JWT_SECRET so it now correctly expects the environment variable to be set, panicking at startup if missing. Added JWT_SECRET to .env.example.
• Issue Nonce table has no periodic cleanup job, causing unbounded row growth in jwt_nonces #823: Created a background Tokio task in main.rs to periodically (every 15 minutes) clean up jwt_nonces that have expired or are older than 7 days and used.
• Issue Add GET /api/v1/health/redis endpoint #580: Implemented the GET /api/v1/health/redis endpoint in handlers/health.rs to check Redis connectivity.

Closes #581
Closes #822
Closes #823
Closes #580

[vercel]

@rhoggs-bot-test-account is attempting to deploy a commit to the oseh-svg's projects Team on Vercel.

A member of the Team first needs to authorize it.

[drips-wave]

@software321dev Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[OSEH-svg]

@software321dev thanks for your contribution.