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/Dockerfile

@@ -0,0 +1,74 @@
+#--------------------------------------------
+# Stage: building TypeScript
+#--------------------------------------------
+FROM node:20 AS build-stage
+
+ENV WORKDIR=/app
+WORKDIR /app
+
+COPY ./package*.json ./
+
+# instaliram pakete
+RUN npm i && npm cache clean --force
+
+COPY ./tsconfig.json ./
+COPY ./src ./src
+RUN npm run build
+
+#--------------------------------------------
+# Stage: instaliram produkcijski node_modules
+#--------------------------------------------
+FROM node:20 AS package-stage
+
+WORKDIR /app
+
+COPY ./package*.json ./
+
+# instaliram SAMO produkcijske
+RUN npm i --omit=dev && npm cache clean --force
+
+#--------------------------------------------
+# Stage: priprema finalnog image-a
+#--------------------------------------------
+FROM gcr.io/distroless/nodejs:20 AS assembly-stage
+
+WORKDIR /app
+
+ARG PORT
+ENV PORT=${PORT}
+
+# (optional) App label to be used in Prometheus (Grafana)
+ARG PROMETHEUS_APP_LABEL
+ENV PROMETHEUS_APP_LABEL=${PROMETHEUS_APP_LABEL}=${PROMETHEUS_APP_LABEL}
+
+# (optional) Prometheus histogram bucket sizes (grafana)
+ARG PROMETHEUS_HISTOGRAM_BUCKETS
+ENV PROMETHEUS_HISTOGRAM_BUCKETS=${PROMETHEUS_HISTOGRAM_BUCKETS}=${PROMETHEUS_HISTOGRAM_BUCKETS}
+
+# CORS settings: kojim domenama dopuštam pristup slikama
+ARG CORS_ALLOWED_ORIGINS
+ENV CORS_ALLOWED_ORIGINS=${CORS_ALLOWED_ORIGINS}
+
+# (optional) IP Address whitelist za metrics i prtg router
+ARG METRICS_ALLOWED_IP_ADDRESSES
+ENV METRICS_ALLOWED_IP_ADDRESSES=${METRICS_ALLOWED_IP_ADDRESSES}
+
+# (optional) uključuje logging u stdout
+ARG DEBUG
+ENV DEBUG=${DEBUG}
+
+# kopiram node-modules
+COPY  --from=package-stage /app/package*.json ./
+COPY  --from=package-stage /app/node_modules ./node_modules
+
+# kopiram buildane datoteke
+COPY  --from=build-stage /app/build ./server
+
+# server vrtim pod ograničenim "nobody" korisnikom
+USER nobody:nobody
+
+HEALTHCHECK --interval=30s --timeout=3s --start-period=10s \
+   CMD ["/nodejs/bin/node", "./server/healthcheck.js"]
+
+# pokrećem server
+CMD ["./server/entry.js"]