Email queue + async sending; legal settings and placeholders

- Add in-memory email queue with rate limiting (MAX_EMAILS_PER_HOUR)
- Bulk send to event attendees now queues and returns immediately
- Frontend shows 'Emails are being sent in the background'
- Legal pages, settings, and placeholders updates

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/lib/legal-placeholders.ts

@@ -0,0 +1,80 @@
+import { getLegalSettingsValues } from '../routes/legal-settings.js';
+
+/**
+ * Strict whitelist of supported placeholders.
+ * Only these placeholders will be replaced in legal page content.
+ * Unknown placeholders remain unchanged.
+ */
+const SUPPORTED_PLACEHOLDERS = new Set([
+  'COMPANY_NAME',
+  'LEGAL_ENTITY_NAME',
+  'RUC_NUMBER',
+  'COMPANY_ADDRESS',
+  'COMPANY_CITY',
+  'COMPANY_COUNTRY',
+  'SUPPORT_EMAIL',
+  'LEGAL_EMAIL',
+  'GOVERNING_LAW',
+  'JURISDICTION_CITY',
+  'CURRENT_YEAR',
+  'LAST_UPDATED_DATE',
+]);
+
+/**
+ * Replace legal placeholders in content using strict whitelist mapping.
+ *
+ * Rules:
+ * - Only supported placeholders are replaced
+ * - Unknown placeholders remain unchanged
+ * - Missing values are replaced with empty string
+ * - No code execution or dynamic evaluation
+ * - Replacement is pure string substitution
+ *
+ * @param content - The markdown/text content containing {{PLACEHOLDER}} tokens
+ * @param updatedAt - The page's updated_at timestamp (for LAST_UPDATED_DATE)
+ * @returns Content with placeholders replaced
+ */
+export async function replaceLegalPlaceholders(
+  content: string,
+  updatedAt?: string
+): Promise<string> {
+  if (!content) return content;
+
+  // Fetch legal settings values from DB
+  const settingsValues = await getLegalSettingsValues();
+
+  // Build the full replacement map
+  const replacements: Record<string, string> = { ...settingsValues };
+
+  // Dynamic values
+  replacements['CURRENT_YEAR'] = new Date().getFullYear().toString();
+
+  if (updatedAt) {
+    try {
+      const date = new Date(updatedAt);
+      if (!isNaN(date.getTime())) {
+        replacements['LAST_UPDATED_DATE'] = date.toLocaleDateString('en-US', {
+          year: 'numeric',
+          month: 'long',
+          day: 'numeric',
+        });
+      } else {
+        replacements['LAST_UPDATED_DATE'] = updatedAt;
+      }
+    } catch {
+      replacements['LAST_UPDATED_DATE'] = updatedAt;
+    }
+  }
+
+  // Replace only whitelisted placeholders using a single regex pass
+  // Matches {{PLACEHOLDER_NAME}} where PLACEHOLDER_NAME is uppercase letters and underscores
+  return content.replace(/\{\{([A-Z_]+)\}\}/g, (match, placeholderName) => {
+    // Only replace if the placeholder is in the whitelist
+    if (!SUPPORTED_PLACEHOLDERS.has(placeholderName)) {
+      return match; // Unknown placeholder - leave unchanged
+    }
+
+    // Return the value or empty string if missing
+    return replacements[placeholderName] ?? '';
+  });
+}