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/.env.example

@@ -0,0 +1,26 @@
+# Worker Configuration
+PULL_INTERVAL=60000
+EMAIL_BUDGET=10
+
+# MongoDB Configuration
+MONGODB_URI=mongodb://localhost:27017/utility-bills
+
+# Mailgun Configuration
+MAILGUN_API_KEY=your-mailgun-api-key-here
+MAILGUN_DOMAIN=rezije.app
+
+# Security
+SHARE_LINK_SECRET=your-secret-key-here
+
+# Server Configuration
+PORT=3000
+
+# Logging
+DEBUG=worker:*,email:*,db:*
+
+# Prometheus Metrics (optional)
+PROMETHEUS_APP_LABEL=email-worker
+PROMETHEUS_HISTOGRAM_BUCKETS=0.1, 0.5, 1, 5, 10
+
+# Environment
+ENV=dev