Implement Rate Limiting

[FerdinandSu]

Description / 描述

English:
Add rate limiting to prevent API abuse:

• Implement request rate limiting per access token
• Use sliding window algorithm
• Add configurable limits (requests per minute/hour)
• Return proper 429 status codes
• Add rate limit headers to responses

中文:
添加速率限制以防止 API 滥用：

• 为每个访问令牌实现请求速率限制
• 使用滑动窗口算法
• 添加可配置的限制（每分钟/小时的请求数）
• 返回正确的 429 状态码
• 向响应添加速率限制头

Difficulty / 难度

Medium / 中等

Estimated Time / 预计时间

4-5 hours

Acceptance Criteria / 验收标准

• Install rate limiting middleware package
• Configure rate limiting in Program.cs
• Add rate limit configuration via environment variables
• Implement per-access-token limiting
• Add rate limit headers (X-RateLimit-*)
• Add documentation
• Add tests for rate limiting

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