feat: implement payment processing, notifications, analytics, and WebSocket support

[SweetBoy-eth]

Summary

This PR implements four major features for the StellFlow backend:

closes #19 - Payment Processing and Transaction Recording

• New files: src/controllers/payment.controller.ts, src/routes/payment.routes.ts, src/validators/payment.schema.ts
• Endpoints: POST /api/payments (record payment), GET /api/payments (list with pagination), GET /api/payments/:id (get by ID), POST /api/payments/verify (verify on Stellar), POST /api/payments/webhook (Stellar confirmation webhook)
• Modified: src/services/stellar.service.ts — added verifyTransaction() for on-chain verification
• Supports duplicate detection via unique txHash, escrow linking, and real-time WebSocket events on confirmation
  closes Implement notification system #20 - Notification System
• New files: src/controllers/notification.controller.ts, src/routes/notification.routes.ts, src/validators/notification.schema.ts
• Endpoints: POST /api/notifications, GET /api/notifications (paginated with type/read filters), GET /api/notifications/unread-count, POST /api/notifications/mark-read (bulk), POST /api/notifications/mark-all-read, GET /api/notifications/:id, DELETE /api/notifications/:id
• Supports four notification types: PAYMENT_RECEIVED, ESCROW_FUNDED, INVOICE_SENT, ESCROW_RELEASED
• Modified: prisma/schema.prisma — updated NotificationType enum to match the four types above
  closes Create dashboard analytics endpoints #21 - Dashboard Analytics Endpoints
• New files: src/controllers/analytics.controller.ts, src/routes/analytics.routes.ts
• Endpoints: GET /api/analytics/overview (total earnings, pending payments, escrow summary), GET /api/analytics/monthly-revenue (chart data), GET /api/analytics/transaction-volume (stats by status), GET /api/analytics/escrow (escrow breakdown), GET /api/analytics/earnings (earnings by period)
• Uses Prisma aggregate, count, and findMany with proper date range filtering and @sum, @AvG, @count operators
  closes Add WebSocket support for real-time updates #22 - WebSocket Support for Real-Time Updates
• New files: src/services/socket.service.ts
• Modified: src/index.ts — integrated Socket.IO server with HTTP server, src/controllers/escrow.controller.ts — broadcasts escrow:stateChange, escrow:funded, escrow:released, escrow:refunded events, src/controllers/invoice.controller.ts — broadcasts invoice:sent, invoice:statusUpdate events, src/controllers/payment.controller.ts — broadcasts payment:confirmed events
• JWT-authenticated WebSocket connections, room-based targeting (user:{userId}, escrow:{escrowId}), join/leave escrow room support
  Supporting Changes
• src/utils/AppError.ts — added BadRequestError class
• src/middleware/auth.ts — exported AuthRequest interface
• src/routes/invoice.routes.ts — fixed import name mismatch (getInvoiceById → getInvoice)
• package.json / package-lock.json — added socket.io dependency
  Branch: feature/payment-processing-notifications-analytics-websocket