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/src/middleware/InitLocalsMiddleware.ts

@@ -0,0 +1,43 @@
+import { Response, Request, NextFunction } from 'express';
+import { AppLocals } from '../types/AppLocals';
+import { requestDurationHistogram, totalRequestCounter } from '../lib/metricsCounters';
+import { SupportedRoutes } from '../types/enums/SupportedRoutes';
+
+/**
+ * Middleware initializes infrastructure objects which will be used throughout the request lifecycle
+ * @param req - Express request object
+ * @param res - Express response object
+ * @param next - Express next middleware function
+ */
+export const InitLocalsMiddleware = (req: Request, res: Response, next: NextFunction) => {
+    try {
+        switch(req.path) {
+            // for metrics routes, no prometheus timer is needed
+            case SupportedRoutes.metricsPath:
+            case '/favicon.ico':
+                // placeholder method to avoid checking if timer is initialized
+                (res.locals as AppLocals).stopPrometheusTimer = (labels) => 0;
+                break;
+            // all other routes get prometheus metrics
+            default:
+                // The request must be processed even if Prometheus does not work
+                // That's why here we wrap the Prometheus calls in try/catch
+                try {
+                    // counting all received requests
+                    totalRequestCounter.inc({ path: req.path });
+
+                    // starting a timer to measure request processing duration
+                    // this timer will be stopped in the route handler
+                    (res.locals as AppLocals).stopPrometheusTimer = requestDurationHistogram.startTimer();
+                } catch(ex:any) {
+                    console.error(ex);
+                }
+                break;
+        }
+
+        next();
+    } catch(ex:any) {
+        console.error('Error in InitLocalsMiddleware:', ex);
+        next(ex);
+    }
+}