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/routes/finalErrorRouter.ts

@@ -0,0 +1,33 @@
+import { ErrorRequestHandler, Request, Response } from "express";
+import { HttpError } from "http-errors";
+import { logError } from '../lib/logger';
+
+/**
+ * Final error handler that executes when an unhandled error occurs.
+ * This prevents the server from crashing and ensures proper response handling.
+ * 
+ * @param err - HTTP error object
+ * @param req - Express request object
+ * @param res - Express response object
+ * @param next - Express next function
+ */
+export const finalErrorRouter:ErrorRequestHandler = async (err:HttpError, req, res, next) => {
+
+    const errorLogText:string = JSON.stringify({ message:err.message, name:err.name, stack:err.stack });
+
+    logError("server error", `${err.status}; n${errorLogText}`);
+
+    // `headersSent` will be TRUE if the router where the error occurred has already sent headers
+    // If we try to set them again, it will throw an error and CRASH THE SERVER - we must prevent this
+    if(!res.headersSent) {
+        res.status(err.status);
+        res.setHeader('Content-Type', "text/html");
+        res.end(`unhandled server error`);
+    } else {
+        // If `end` hasn't been called - call it to finish processing the request
+        // Otherwise the connection will remain open until timeout
+        if(!res.writableEnded) {
+            res.end();
+        }
+    }
+};