Add rate limiting per user

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Summary

Implement per-user rate limiting to prevent abuse, even from allowed users.

Implementation Options

1. In-memory - Simple Map with timestamps, resets on restart
2. Redis-based - Persistent across restarts, uses existing Redis connection

Suggested Behavior

• Track request timestamps per Discord user ID
• Configurable via env: RATE_LIMIT_REQUESTS=5, RATE_LIMIT_WINDOW_MS=60000
• Default: 5 requests per 60 seconds
• Respond with rate limit message when exceeded

Priority

Medium - implement after user/channel allowlist (#1)

Related

• Depends on Add user/channel allowlist for access control #1 (allowlist should be checked first, rate limiting second)