feat: implement audit logging, Stellar wallet verification, and enhanced error handling

[SweetBoy-eth]

Summary

This PR implements three major features for the StellFlow backend:

• closes Add audit logging for sensitive operations #29 - Audit Logging for Sensitive Operations
• New files: src/services/audit.service.ts, src/controllers/audit.controller.ts, src/routes/audit.routes.ts
• Prisma schema: Added AuditLog model with AuditAction enum covering 18 action types (PAYMENT_CREATED, ESCROW_FUNDED, ACCOUNT_UPDATED, WALLET_VERIFIED, etc.)
• Audit service: logAudit() and logAuditFromRequest() functions capture user ID, action, resource, IP address, user agent, and custom metadata
• Audit endpoints: GET /api/audit-logs (admin only), GET /api/audit-logs/my (user's own logs), GET /api/audit-logs/:id
• Controllers updated with audit logging:
  • escrow.controller.ts — ESCROW_CREATED, ESCROW_FUNDED, ESCROW_RELEASED, ESCROW_REFUNDED
  • payment.controller.ts — PAYMENT_CREATED, PAYMENT_VERIFIED, PAYMENT_FAILED
  • invoice.controller.ts — INVOICE_CREATED, INVOICE_UPDATED, INVOICE_STATUS_CHANGED, INVOICE_DELETED
  • user.controller.ts — ACCOUNT_UPDATED, ACCOUNT_DELETED
  • auth.controller.ts — USER_REGISTERED, USER_LOGIN
    closes Implement Stellar wallet verification endpoint #28 - Stellar Wallet Verification Endpoint
• New files: src/services/wallet-verification.service.ts, src/controllers/wallet.controller.ts, src/routes/wallet.routes.ts, src/validators/wallet.schema.ts
• Challenge-response flow:
  1. POST /api/wallet/challenge — generates a random 32-byte hex challenge with 5-minute expiry
  2. POST /api/wallet/verify — verifies signed challenge, validates Stellar account exists on network, links wallet to user
  3. POST /api/wallet/unlink — removes linked wallet from account
• Validates Stellar account existence via Horizon API before linking
• Prevents wallet address conflicts (unique per user)
• Records audit logs for wallet verification and unlinking
  closes Add error handling middleware and custom error classes #27 - Error Handling Middleware and Custom Error Classes
• Enhanced src/utils/AppError.ts:
  • Added code field to AppError base class for machine-readable error codes
  • Added RateLimitError (429) and InternalError (500) classes
  • All error classes now include standardized error codes: NOT_FOUND, UNAUTHORIZED, FORBIDDEN, VALIDATION_ERROR, CONFLICT, BAD_REQUEST, RATE_LIMIT_EXCEEDED, INTERNAL_ERROR
• Enhanced src/middleware/errorHandler.ts:
  • Structured JSON logging with timestamp, method, path, IP, userId, statusCode, message, code, and stack trace
  • Consistent error response format with success, error.message, and error.code
  • Environment-aware stack trace inclusion (production vs development)
    Branch: feature/audit-logging-wallet-verification-error-handling