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/src/workRunner.ts

@@ -2,7 +2,7 @@ import { failedRequestCounter, requestDurationHistogram, successfulRequestCounte
 import { coalesce } from "./lib/initTools";
 import { createLogger } from "./lib/logger";
 import { serializeError } from "./lib/serializeError";
-import { doWork } from "./exampleWorker";
+import { doWork } from "./emailWorker";
 
 /** time between two pull operations */
 const PULL_INTERVAL = parseInt(coalesce(process.env.PULL_INTERVAL, "10000"));