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/lib/dbClient.ts

@@ -0,0 +1,53 @@
+import { MongoClient, Db } from 'mongodb';
+import { createLogger } from './logger';
+
+const log = createLogger("db:client");
+
+let client: MongoClient | null = null;
+let db: Db | null = null;
+
+/**
+ * Connect to MongoDB
+ * @returns Database instance
+ */
+export async function connectToDatabase(): Promise<Db> {
+    if (!process.env.MONGODB_URI) {
+        throw new Error('MONGODB_URI environment variable is not set');
+    }
+
+    if (db) {
+        log('Reusing existing database connection');
+        return db;
+    }
+
+    log('Creating new database connection');
+    client = new MongoClient(process.env.MONGODB_URI);
+    await client.connect();
+    db = client.db("utility-bills");
+
+    log('Connected to database');
+    return db;
+}
+
+/**
+ * Disconnect from MongoDB
+ */
+export async function disconnectFromDatabase(): Promise<void> {
+    if (client) {
+        log('Disconnecting from database');
+        await client.close();
+        client = null;
+        db = null;
+        log('Disconnected from database');
+    }
+}
+
+/**
+ * Get current database instance (must call connectToDatabase first)
+ */
+export function getDatabase(): Db {
+    if (!db) {
+        throw new Error('Database not connected. Call connectToDatabase() first.');
+    }
+    return db;
+}