Issue #3: Backend API Endpoints (Node.js)

[achneerov]

Description: Implement three core API endpoints for transaction processing and MFA verification.
Endpoints:

1. processTransaction(hashCC, amount, location, merchantApiKey, emailAddress)
   Logic Flow:
2. Validate merchant API key
3. Check if user exists in system
4. If user doesn't exist → return AUTH_REQUIRED
5. If user exists → evaluate transaction rules
6. Return response based on rules evaluation
   Response Types:

• SUCCESS: Transaction approved
• AUTH_REQUIRED: Include array of enabled auth method IDs (e.g., [1, 3] for email and phone)
• FAILURE: Transaction denied

2. requestCode(hashCC, authMode)
   Logic Flow:
3. Identify authentication method from authMode parameter
4. Generate random 6-digit code
5. Update userTable.authCode with generated code
6. Send code via selected method (email, SMS, etc.)
7. Return confirmation status
8. verifyMFA(hashCC, code)
   Logic Flow:
9. Retrieve stored auth code from userTable.authCode
10. Compare with submitted code
11. Return SUCCESS if match, AUTH_REQUIRED if mismatch, FAILURE for errors
    Technical Requirements:

• All endpoints return standardized status codes
• Implement proper error handling
• Log all attempts to mfa_events table
• Support multiple authentication methods (email, phone, etc.)