feat: implement MailGun webhook service for logging email events

Implemented a production-ready TypeScript/Express.js service to receive
and log MailGun webhook events (delivered, failed, opened, clicked, etc.).

Key features:
- Webhook endpoint (POST /webhook) with comprehensive event logging
- Full TypeScript type definitions for all MailGun event types
- Prometheus metrics integration for monitoring
- Health check endpoint (GET /ping)
- Comprehensive Jest test suite with 87.76% coverage
- Docker containerization with build scripts

Removed template/example code:
- All SQL/MSSQL dependencies and related code
- Example auth router and middleware
- PRTG metrics support (simplified to Prometheus only)
- Unused middleware (CORS, IP whitelist, request parsing/validation)
- Template documentation (kept only MailGun webhook API spec)

The service is clean, minimal, and focused solely on receiving and
logging MailGun webhook events to the console.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

mailgun-webhook/tests/__mocks__/logger.ts

@@ -0,0 +1,18 @@
+/**
+ * Mock implementation of the logger module for testing
+ */
+
+export const logServer = jest.fn();
+export const logHealthCheck = jest.fn();
+export const logError = jest.fn();
+export const logWarn = jest.fn();
+export const logInfo = jest.fn();
+
+// Helper function to reset all mocks
+export const resetAllLoggerMocks = () => {
+    logServer.mockClear();
+    logHealthCheck.mockClear();
+    logError.mockClear();
+    logWarn.mockClear();
+    logInfo.mockClear();
+};