Implement API Rate Limiting

## Overview
Add rate limiting to API endpoints to prevent abuse and ensure fair resource usage.

## Business Value
Rate limiting protects the system from abuse and ensures fair resource distribution among users.

## Current State
- No rate limiting implemented
- Risk of API abuse
- No quota enforcement

## Subtasks
- [ ] Design rate limiting strategy
- [ ] Implement rate limiting middleware
- [ ] Add per-user limits
- [ ] Add per-organization limits
- [ ] Implement rate limit headers
- [ ] Add rate limit configuration
- [ ] Create rate limit monitoring

## Implementation Steps
1. Create backend/internal/middleware/ratelimit.go
2. Implement token bucket algorithm
3. Add rate limit storage (Redis)
4. Add rate limit headers to responses
5. Create rate limit configuration

## Acceptance Criteria
- [ ] Rate limits are enforced per user
- [ ] Rate limits are enforced per organization
- [ ] Rate limit headers are included
- [ ] Rate limit errors are clear
- [ ] Configuration is flexible
- [ ] Rate limits are monitored

## Estimated Effort
- Hours: 12-16
- Complexity: Medium