feat: refactor email-worker to use HTML email templates

Replace inline HTML in email notifications with professional HTML templates
for better email client compatibility and consistent branding.

Changes:
- Created emailTemplates.ts utility for loading and rendering templates
  - Template caching for performance
  - Variable substitution with ${variable} syntax
  - Warning logging for unreplaced variables

- Updated sendVerificationRequests to use email-validation template
  - Variables: location.tenantName, ownerName, location.name, shareId

- Updated sendRentDueNotifications to use rent-due template
  - Fetches user settings for owner name and currency
  - Calculates rent due date from yearMonth and rentDueDay
  - Formats rent amount (converts cents to display format)
  - Variables: location.tenantName, location.name, rentDueDate,
    rentAmount, currency, ownerName, shareId

- Updated sendUtilityBillsNotifications to use util-bills-due template
  - Calculates total amount from all bills
  - Fetches user settings for owner name and currency
  - Variables: location.tenantName, location.name, totalAmount,
    currency, ownerName, shareId

- Fixed ObjectId type mismatches in MongoDB operations

All emails now feature:
- Responsive 3-column layout
- rezije.app branding with logo
- Professional typography and spacing
- Unsubscribe links
- Email client compatible table-based layouts

🤖 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

@@ -2,6 +2,7 @@ import { Db, ObjectId } from 'mongodb';
 import { BillingLocation, EmailStatus, UserSettings, generateShareId } from '@evidencija-rezija/shared-code';
 import { sendEmail } from './mailgunService';
 import { createLogger } from './logger';
+import { loadAndRender } from './emailTemplates';
 
 const log = createLogger("email:senders");
 
@@ -53,23 +54,12 @@ export async function sendVerificationRequests(db: Db, budget: number): Promise<
         const ownerName = userSettings?.ownerName || '';
         const shareId = generateShareId(location._id.toString());
 
-        const html = `
+        const html = loadAndRender('email-validation', {
-<p>Hello ${location.tenantName || 'there'}!</p>
+            'location.tenantName': location.tenantName || 'there',
-
+            'ownerName': ownerName,
-<p>You have received this e-mail because your landlord <strong>${ownerName}</strong> wants to send you rent and utility bills invoices for ${location.name} via rezije.app</p>
+            'location.name': location.name,
-
+            'shareId': shareId
-<p><strong>rezije.app</strong> is an online app which helps property owners to manage expenses related to properties they lease.</p>
+        });
-
-<p>Before the app can start sending you rent due and utility bills emails we need your verification.</p>
-
-<p>To verify that you want to receive these notifications please click on the following link: <a href="https://rezije.app/email/verify/${shareId}">Verify</a></p>
-
-<p>You can ignore this email if you don't want to receive notifications. You can also unsubscribe at any time using the link included in every notification email you receive.</p>
-
-<p>Thank you!</p>
-
-<a href="https://rezije.app" target="_blank">rezije.app</a>
-        `;
 
         const success = await sendEmail({
             to: location.tenantEmail,
@@ -80,7 +70,7 @@ export async function sendVerificationRequests(db: Db, budget: number): Promise<
         // Update location status
         const newStatus = success ? EmailStatus.VerificationPending : EmailStatus.VerificationFailed;
         await db.collection('lokacije').updateOne(
-            { _id: location._id },
+            { _id: new ObjectId(location._id as any) },
             { $set: { tenantEmailStatus: newStatus } }
         );
 
@@ -149,21 +139,29 @@ export async function sendRentDueNotifications(db: Db, budget: number): Promise<
             continue;
         }
 
+        // Fetch user settings
+        const userSettings = await db.collection<UserSettings>('userSettings')
+            .findOne({ userId: location.userId });
+
+        const ownerName = userSettings?.ownerName || '';
         const shareId = generateShareId(location._id.toString());
 
-        const html = `
+        // Format rent due date
-<p>Hello ${location.tenantName || 'there'}!</p>
+        const rentDueDate = `${location.yearMonth.month}/${location.rentDueDay}/${location.yearMonth.year}`;
 
-<p>Your rent for the apartment ${location.name} is due today.</p>
+        // Format rent amount (convert from cents to display format)
+        const rentAmount = location.rentAmount ? (location.rentAmount / 100).toFixed(2) : '0.00';
+        const currency = userSettings?.currency || 'EUR';
 
-<p>For details and payment options please click the following link: <a href="https://rezije.app/share/location/rent/${shareId}">Rent details</a></p>
+        const html = loadAndRender('rent-due', {
-
+            'location.tenantName': location.tenantName || 'there',
-<p>Thank you!</p>
+            'location.name': location.name,
-
+            'rentDueDate': rentDueDate,
-<p><a href="https://rezije.app" target="_blank">rezije.app</a></p>
+            'rentAmount': rentAmount,
-
+            'currency': currency,
-<p style="font-size:.7em">If you do no longer want to receive these notifications please click the following link: <a href="https://rezije.app/email/unsubscribe/${shareId}">Unsubscribe</a></p>
+            'ownerName': ownerName,
-        `;
+            'shareId': shareId
+        });
 
         const success = await sendEmail({
             to: location.tenantEmail,
@@ -174,7 +172,7 @@ export async function sendRentDueNotifications(db: Db, budget: number): Promise<
         // Update location status
         const newStatus = success ? 'sent' : 'failed';
         await db.collection('lokacije').updateOne(
-            { _id: location._id },
+            { _id: new ObjectId(location._id as any) },
             { $set: { rentDueNotificationStatus: newStatus } }
         );
 
@@ -235,21 +233,28 @@ export async function sendUtilityBillsNotifications(db: Db, budget: number): Pro
             continue;
         }
 
+        // Fetch user settings
+        const userSettings = await db.collection<UserSettings>('userSettings')
+            .findOne({ userId: location.userId });
+
+        const ownerName = userSettings?.ownerName || '';
         const shareId = generateShareId(location._id.toString());
 
-        const html = `
+        // Calculate total amount from all bills
-<p>Hello ${location.tenantName || 'there'}!</p>
+        const totalAmountCents = (location.bills || []).reduce((sum, bill) => {
+            return sum + (bill.payedAmount || 0);
+        }, 0);
+        const totalAmount = (totalAmountCents / 100).toFixed(2);
+        const currency = userSettings?.currency || 'EUR';
 
-<p>Your utility bills for the apartment ${location.name} are due today.</p>
+        const html = loadAndRender('util-bills-due', {
-
+            'location.tenantName': location.tenantName || 'there',
-<p>For details and payment options please click the following link: <a href="https://rezije.app/share/location/bills/${shareId}">Utility Bills</a></p>
+            'location.name': location.name,
-
+            'totalAmount': totalAmount,
-<p>Thank you!</p>
+            'currency': currency,
-
+            'ownerName': ownerName,
-<p><a href="https://rezije.app" target="_blank">rezije.app</a></p>
+            'shareId': shareId
-
+        });
-<p style="font-size:.7em">If you do no longer want to receive these notifications please click the following link: <a href="https://rezije.app/email/unsubscribe/${shareId}">Unsubscribe</a></p>
-        `;
 
         const success = await sendEmail({
             to: location.tenantEmail,
@@ -260,7 +265,7 @@ export async function sendUtilityBillsNotifications(db: Db, budget: number): Pro
         // Update location status
         const newStatus = success ? 'sent' : 'failed';
         await db.collection('lokacije').updateOne(
-            { _id: location._id },
+            { _id: new ObjectId(location._id as any) },
             { $set: { billFwdStatus: newStatus } }
         );
 

email-worker/src/lib/emailTemplates.ts

@@ -0,0 +1,87 @@
+import fs from 'fs';
+import path from 'path';
+import { createLogger } from './logger';
+
+const log = createLogger('email:templates');
+
+// Cache for loaded templates
+const templateCache = new Map<string, string>();
+
+/**
+ * Template variable type for type-safe template rendering
+ */
+export type TemplateVariables = {
+    [key: string]: string | number | undefined;
+};
+
+/**
+ * Load an email template from the templates directory
+ * @param templateName Name of the template file (without extension)
+ * @param language Language code (default: 'en')
+ * @returns Template content as string
+ */
+export function loadTemplate(templateName: string, language: string = 'en'): string {
+    const cacheKey = `${templateName}--${language}`;
+
+    // Check cache first
+    if (templateCache.has(cacheKey)) {
+        log(`Using cached template: ${cacheKey}`);
+        return templateCache.get(cacheKey)!;
+    }
+
+    // Construct template file path
+    const templateFileName = `email-template--${templateName}--${language}.html`;
+    const templatePath = path.join(__dirname, '../../email-templates', templateFileName);
+
+    try {
+        const content = fs.readFileSync(templatePath, 'utf-8');
+        templateCache.set(cacheKey, content);
+        log(`Loaded template: ${templateFileName}`);
+        return content;
+    } catch (error) {
+        log(`Failed to load template ${templateFileName}: ${error}`);
+        throw new Error(`Template not found: ${templateFileName}`);
+    }
+}
+
+/**
+ * Render a template by replacing variables
+ * @param template Template content
+ * @param variables Object with variable values
+ * @returns Rendered HTML string
+ */
+export function renderTemplate(template: string, variables: TemplateVariables): string {
+    let rendered = template;
+
+    // Replace all ${variable} occurrences
+    for (const [key, value] of Object.entries(variables)) {
+        if (value !== undefined) {
+            const regex = new RegExp(`\\$\\{${key}\\}`, 'g');
+            rendered = rendered.replace(regex, String(value));
+        }
+    }
+
+    // Log warning if there are unreplaced variables
+    const unreplacedMatches = rendered.match(/\$\{[^}]+\}/g);
+    if (unreplacedMatches) {
+        log(`Warning: Unreplaced variables in template: ${unreplacedMatches.join(', ')}`);
+    }
+
+    return rendered;
+}
+
+/**
+ * Load and render an email template in one step
+ * @param templateName Name of the template file (without extension)
+ * @param variables Object with variable values
+ * @param language Language code (default: 'en')
+ * @returns Rendered HTML string
+ */
+export function loadAndRender(
+    templateName: string,
+    variables: TemplateVariables,
+    language: string = 'en'
+): string {
+    const template = loadTemplate(templateName, language);
+    return renderTemplate(template, variables);
+}