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/helpers/mockWebhookEvent.ts

@@ -0,0 +1,115 @@
+/**
+ * Helper for creating mock MailGun webhook events for testing
+ */
+
+import {
+  MailgunWebhookEvent,
+  MailgunDeliveredEvent,
+  MailgunFailedEvent,
+  MailgunOpenedEvent,
+  MailgunClickedEvent,
+  MailgunBouncedEvent,
+  MailgunComplainedEvent,
+  MailgunUnsubscribedEvent
+} from '../../src/types/MailgunWebhookEvent';
+
+/**
+ * Base event data common to all webhook events
+ */
+const baseEventData = {
+  timestamp: '1234567890',
+  token: 'mock-token-123',
+  signature: 'mock-signature-abc123',
+  recipient: 'user@example.com',
+  domain: 'mail.example.com',
+  'message-id': '<20240101120000.1.ABC123@mail.example.com>',
+  'Message-Id': '<20240101120000.1.ABC123@mail.example.com>'
+};
+
+/**
+ * Creates a mock 'delivered' event
+ */
+export const createMockDeliveredEvent = (): MailgunDeliveredEvent => ({
+  ...baseEventData,
+  event: 'delivered',
+  'message-headers': JSON.stringify([
+    ['Subject', 'Test Email'],
+    ['From', 'sender@example.com']
+  ])
+});
+
+/**
+ * Creates a mock 'failed' event
+ */
+export const createMockFailedEvent = (): MailgunFailedEvent => ({
+  ...baseEventData,
+  event: 'failed',
+  severity: 'permanent',
+  reason: 'bounce',
+  notification: 'User inbox is full'
+});
+
+/**
+ * Creates a mock 'opened' event
+ */
+export const createMockOpenedEvent = (): MailgunOpenedEvent => ({
+  ...baseEventData,
+  event: 'opened',
+  city: 'San Francisco',
+  country: 'US',
+  'device-type': 'desktop',
+  'client-os': 'macOS',
+  'client-name': 'Apple Mail',
+  ip: '192.168.1.100'
+});
+
+/**
+ * Creates a mock 'clicked' event
+ */
+export const createMockClickedEvent = (): MailgunClickedEvent => ({
+  ...baseEventData,
+  event: 'clicked',
+  url: 'https://example.com/link',
+  city: 'New York',
+  country: 'US',
+  'device-type': 'mobile',
+  'client-os': 'iOS',
+  'client-name': 'Gmail',
+  ip: '192.168.1.101'
+});
+
+/**
+ * Creates a mock 'bounced' event
+ */
+export const createMockBouncedEvent = (): MailgunBouncedEvent => ({
+  ...baseEventData,
+  event: 'bounced',
+  code: '550',
+  error: 'User not found',
+  notification: 'The email account that you tried to reach does not exist'
+});
+
+/**
+ * Creates a mock 'complained' event
+ */
+export const createMockComplainedEvent = (): MailgunComplainedEvent => ({
+  ...baseEventData,
+  event: 'complained'
+});
+
+/**
+ * Creates a mock 'unsubscribed' event
+ */
+export const createMockUnsubscribedEvent = (): MailgunUnsubscribedEvent => ({
+  ...baseEventData,
+  event: 'unsubscribed'
+});
+
+/**
+ * Creates an invalid event missing required fields
+ */
+export const createInvalidEvent = () => ({
+  timestamp: '1234567890',
+  token: 'mock-token-123'
+  // Missing 'event' and 'recipient' fields
+});