refactor: migrate email-worker to use shared-code package

Update email-worker to use @evidencija-rezija/shared-code for common types
and utilities instead of maintaining duplicate copies.

Changes:
- Add shared-code dependency to package.json
- Update imports in emailSenders.ts to use shared-code
- Remove duplicate db-types.ts and shareChecksum.ts files

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

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

email-worker/src/lib/emailSenders.ts

@@ -1,6 +1,5 @@
 import { Db, ObjectId } from 'mongodb';
-import { BillingLocation, EmailStatus, UserSettings } from '../types/db-types';
-import { generateShareId } from './shareChecksum';
+import { BillingLocation, EmailStatus, UserSettings, generateShareId } from '@evidencija-rezija/shared-code';
 import { sendEmail } from './mailgunService';
 import { createLogger } from './logger';
 

email-worker/src/lib/shareChecksum.ts

@@ -1,36 +0,0 @@
-import crypto from 'crypto';
-
-/**
- * Checksum length in hex characters (16 chars = 64 bits of entropy)
- */
-export const CHECKSUM_LENGTH = 16;
-
-/**
- * Generate share link checksum for location
- * Uses HMAC-SHA256 for cryptographic integrity
- *
- * SECURITY: Prevents location ID enumeration while allowing stateless validation
- */
-export function generateShareChecksum(locationId: string): string {
-    const secret = process.env.SHARE_LINK_SECRET;
-
-    if (!secret) {
-        throw new Error('SHARE_LINK_SECRET environment variable not configured');
-    }
-
-    return crypto
-        .createHmac('sha256', secret)
-        .update(locationId)
-        .digest('hex')
-        .substring(0, CHECKSUM_LENGTH);
-}
-
-/**
- * Generate combined location ID with checksum appended
- * @param locationId - The MongoDB location ID (24 chars)
- * @returns Combined ID: locationId + checksum (40 chars total)
- */
-export function generateShareId(locationId: string): string {
-    const checksum = generateShareChecksum(locationId);
-    return locationId + checksum;
-}