Implement subscription API rate limiting and usage metering

[Itodo-S]

Description

Build a metering system that tracks API usage per subscription tier and enforces rate limits with soft warnings and hard caps.

Changes

• Usage metering per API key with hourly/daily/monthly counters with automatic reset
• Tier-based rate limits (free: 100/h, pro: 1000/h, enterprise: custom) with burst and concurrent limits
• Soft limit warning at 80% and 95% usage thresholds
• Hard limit enforcement with 429 response including retry-after headers
• Usage analytics dashboard data with p95/p99 latency, error rates, and top endpoints
• Automatic tier upgrade recommendation on sustained overuse

Files Changed

• backend/services/rateLimitingService.ts - Core rate limiting service with RateLimitingService class
• backend/services/index.ts - Service export
• src/types/rateLimiting.ts - Rate limiting types, tier configurations, and constants

Closes #369

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[Smartdevs17]

@Itodo-S merge conflicts