Add Discord rate limiting with exponential backoff retry mechanism

### Is your feature request related to a problem?

- [x] Yes, it is related to a problem

### Describe the feature you'd like

## Problem
The Discord bot lacks proper rate limit handling:
- **429 errors** occur when hitting Discord API rate limits
- **No recovery mechanism** - errors just propagate to user
- **Poor user experience** - message send failures without auto-retry
- **Silent failures** - no logging of rate limit incidents

## Current Behavior

User command → Bot tries to send → 429 Error → User sees "something went wrong"

## Desired Behavior

User command → Bot tries to send → 429 Error → Auto-retry with exponential backoff → Success

## Solution Overview
Implement a robust rate limiting system with:
1. Rate limit tracking per Discord endpoint/bucket
2. Exponential backoff retry logic (2^n + jitter)
3. Command queueing during rate limits
4. Comprehensive logging and metrics
5. Graceful degradation with user-friendly messages

## Implementation Details

### Component 1: Rate Limiter Core
- Create `backend/rate_limiter.py`
- Implement `DiscordRateLimiter` class with:
- `wait_if_rate_limited()` - Check if endpoint is rate limited
- `execute_with_retry()` - Execute with automatic retry
- Exponential backoff: 2^attempt × retry_after + jitter
- Maximum 3 retry attempts

### Component 2: Discord Client Wrapper
- Create `backend/discord_client.py`
- Implement `EnhancedDiscordClient` class with:
- `send_message_with_retry()` - Send with rate limit handling
- `edit_message_with_retry()` - Edit with rate limit handling
- Error handling for 429 responses
- User-friendly error messages

### Component 3: Redis Integration
- Use Redis to track rate limit state across bot instances
- Store: `discord_ratelimit:bucket:remaining`, `discord_ratelimit_reset:bucket`
- Per-endpoint tracking (channels, reactions, etc.)

## Files to Create
backend/
├── rate_limiter.py (180 lines) - Core rate limiting logic
├── discord_client.py (250 lines) - Enhanced Discord client
└── discord_utils.py (80 lines) - Helper utilities
tests/
├── test_rate_limiter.py (200 lines) - Rate limiter tests
├── test_discord_client.py (150 lines) - Client tests
└── fixtures/discord_mocks.py (100 lines) - Test fixtures
docs/
└── DISCORD_RATE_LIMITING.md (150 lines) - Usage documentation

## Integration Points
- Update `backend/bot.py` to use enhanced client (+45 lines)
- Add `redis>=4.0.0` to `requirements.txt`
- Set environment: `REDIS_URL=redis://localhost:6379`

## Testing
- 12+ unit tests for rate limiter logic
- Integration tests with mock Discord API
- Exponential backoff calculation verification
- Retry exhaustion handling
- Performance tests (minimal overhead when not rate limited)

## Acceptance Criteria
- [ ] Rate limits properly detected and handled
- [ ] Automatic retry with exponential backoff
- [ ] Maximum 3 retry attempts configurable
- [ ] Command queueing during rate limits
- [ ] Rate limit incidents logged with details
- [ ] Zero performance impact when not rate limited
- [ ] 90%+ test coverage
- [ ] No merge conflicts
- [ ] All existing tests still pass
- [ ] Documentation included

## Performance Requirements
- Overhead when NOT rate limited: <1ms per request
- Successful retry adds latency only when rate limited
- Redis operations are async and non-blocking
- No impact on normal message flow

## Migration Guide
For deployments using this:
1. Install Redis: `pip install redis>=4.0.0`
2. Set `REDIS_URL` environment variable
3. Start Redis service
4. Update code to use `EnhancedDiscordClient`
5. Test in staging first
6. Deploy with monitoring

## References
- Discord Rate Limit Docs: https://discord.com/developers/docs/topics/rate-limits
- Exponential Backoff: https://en.wikipedia.org/wiki/Exponential_backoff
- Similar implementation in AWS SDK

## Related Issues
Fixes: Bot resilience failures when hitting Discord limits
Related to: #48 (general bot reliability)

## Nice to Have (Future)
- Prometheus metrics for rate limit tracking
- Rate limit prediction using historical data
- Per-guild rate limiting
- Analytics dashboard

### Record

- [x] I agree to follow this project's Code of Conduct
- [x] I want to work on implementing this feature