Implement calendar billing scheduling and API rate limiting

[Itodo-S]

Description

This PR combines work for two issues to resolve merge conflicts.

#367 - Subscription Payment Scheduling with Calendar Billing

• Calendar-based billing schedule visualization
• One-time scheduled payments beyond recurring
• Calendar export (iCal, Google Calendar, Outlook)
• Schedule conflict detection
• Schedule modification with prorated adjustment
• Push notification for upcoming payments

#369 - API Rate Limiting and Usage Metering

• 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

• src/screens/CalendarIntegrationScreen.tsx - Calendar UI and integration screen
• src/services/calendarService.ts - Core calendar service with export logic
• src/store/calendarStore.ts - Calendar state management
• src/types/calendar.ts - Calendar types and interfaces
• src/types/subscription.ts - Added timezone field for calendar
• backend/services/rateLimitingService.ts - Core rate limiting service
• backend/services/index.ts - Service exports
• src/types/rateLimiting.ts - Rate limiting types and constants

Closes #367
Closes #369