fix: replace blocking Redis client with async implementation

[Prinxeeee]

Use async Redis client for non-blocking operations

Problem
The synchronous redis.Redis client was blocking the asyncio event loop on every token check and cache update, stalling concurrent requests.

Changes

• redis_db.py: switched to redis.asyncio.Redis and made remove_cache async
• login.py: added await to all Redis calls

Bonus fix

• fixed duplicate revocation check in /Refresh (was checking the same condition twice)

Notes

• Existing behavior is preserved (TTL handling and token revocation logic unchanged)
• Import name redis is kept to avoid a larger diff across the codebase