feat: Implement input validation, CORS, fallback chains, and revenue forecasting

[akansmafengadamu]

Summary

This PR implements four major features that address issues #771, #772, #773, and #774.

Changes

Issue #771: Input Validation with Zod Schemas and Sanitization

• Added �alidationMiddleware.ts with Zod schema validation
• XSS sanitization for string inputs
• SQL injection prevention patterns
• File upload validation with configurable limits
• Request body size limits
• Added schemas.ts with reusable Zod schemas for all API endpoints
• Comprehensive documentation in docs/VALIDATION.md

Issue #772: CORS Policy Management with Dynamic Origin Whitelisting

• Added corsMiddleware.ts with dynamic CORS management
• Per-tenant origin whitelisting with wildcard support
• Preflight request caching for performance
• CORS violation logging and analytics
• Express-style middleware factory
• Documentation in docs/CORS.md and developer-portal/docs/cors-guide.md

Issue #773: Subscription Payment Method Fallback Chains

• Added allbackChainService.ts with automatic fallback logic
• Configurable gateway chains per merchant
• Retry attempts with configurable delays
• Fallback notifications for success/failure events
• Comprehensive fallback history and analytics
• Documentation in docs/FALLBACK_CHAINS.md

Issue #774: Subscription Analytics with Revenue Forecasting

• Added
  evenueForecastService.ts with multiple forecasting models
• Linear regression, exponential smoothing, and moving average models
• Trend analysis with direction, strength, and growth rate
• Forecast accuracy tracking (MAE, MAPE, RMSE, R²)
• Visualization data generation for charting
• Alert generation for revenue declines and spikes
• Documentation in docs/REVENUE_FORECASTING.md

Files Changed

• �ackend/services/shared/validationMiddleware.ts (new)
• �ackend/services/shared/schemas.ts (new)
• �ackend/services/shared/corsMiddleware.ts (new)
• �ackend/services/shared/index.ts (updated exports)
• �ackend/services/payment/domain/fallbackChainService.ts (new)
• �ackend/services/payment/index.ts (updated exports)
• �ackend/services/analytics/revenueForecastService.ts (new)
• �ackend/services/analytics/index.ts (updated exports)
• docs/VALIDATION.md (new)
• docs/CORS.md (new)
• docs/FALLBACK_CHAINS.md (new)
• docs/REVENUE_FORECASTING.md (new)
• developer-portal/docs/cors-guide.md (new)

Closes #771
Closes #772
Closes #773
Closes #774

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