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/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);
+}