feat: implement email notification worker with Mailgun integration

- Add MongoDB connection module for database access
- Implement Mailgun email service for sending notifications
- Add shareChecksum utility for generating secure share links
- Implement three email sender functions:
  - Email verification requests (highest priority)
  - Rent due notifications (CET timezone)
  - Utility bills due notifications
- Create main email worker with budget-based email sending
- Add environment variables for configuration
- Install dependencies: mongodb, mailgun.js, form-data
- Update package.json description to reflect email worker purpose
- Add .env.example with all required configuration

The worker processes emails in priority order and respects a configurable
budget to prevent overwhelming the mail server. All database operations are
atomic and updates are performed immediately after each email send.

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

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

email-worker/package.json

@@ -1,7 +1,7 @@
 {
   "name": "email-worker",
   "version": "0.1.0",
-  "description": "Background worker service with HTTP health monitoring and metrics collection",
+  "description": "Email notification worker service for sending verification requests, rent due notices, and utility bills notifications",
   "main": "entry.ts",
   "scripts": {
     "start": "nodemon ./src/entry.ts",
@@ -14,7 +14,10 @@
   "dependencies": {
     "debug": "^2.6.9",
     "express": "^4.18.2",
+    "form-data": "^4.0.5",
     "http-errors": "^1.7.2",
+    "mailgun.js": "^12.4.1",
+    "mongodb": "^7.0.0",
     "node-fetch": "^2.6.7",
     "prom-client": "^14.0.1",
     "stoppable": "^1.1.0"
@@ -25,6 +28,7 @@
     "@types/express": "^4.17.13",
     "@types/http-errors": "^1.8.1",
     "@types/jest": "^29.2.5",
+    "@types/mongodb": "^4.0.6",
     "@types/node": "^16.10.2",
     "@types/node-fetch": "^2.6.2",
     "@types/stoppable": "^1.1.1",
@@ -46,4 +50,4 @@
       "@babel/preset-typescript"
     ]
   }
-}
+}