Implement Redis-based shared rate limit store

[testtest126]

Implement a Redis-based shared counter store for the /auth/* rate limiter to support multi-instance deployments.

This solution would:

• Replace the current in-memory per-process rate limiter with a shared Redis store
• Ensure consistent rate limits across all instances
• Prevent the effective limit from multiplying by the number of instances
• Persist rate limit windows across instance restarts

Related to: #79