fix: Use userEmailAccounts join table for email account lookup

The emailAccounts table doesn't have a userId column - it uses a
many-to-many relationship through userEmailAccounts table.

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

src/services/ai-kurzy/certificate-email.service.js

@@ -1,7 +1,7 @@
 import fs from 'fs/promises';
 import path from 'path';
 import { db } from '../../config/database.js';
-import { prilohy, registracie, ucastnici, kurzy, emailAccounts } from '../../db/schema.js';
+import { prilohy, registracie, ucastnici, kurzy, emailAccounts, userEmailAccounts } from '../../db/schema.js';
 import { eq, and } from 'drizzle-orm';
 import { NotFoundError, BadRequestError } from '../../utils/errors.js';
 import { logger } from '../../utils/logger.js';
@@ -462,17 +462,26 @@ export const sendCertificateEmail = async (prilohaId, userId) => {
     throw new BadRequestError('Účastník nemá zadaný email');
   }
 
-  // Get email account for sending
-  const [emailAccount] = await db
-    .select()
-    .from(emailAccounts)
-    .where(and(eq(emailAccounts.userId, userId), eq(emailAccounts.isActive, true)))
+  // Get email account for sending (through userEmailAccounts join table)
+  const [emailAccountResult] = await db
+    .select({
+      id: emailAccounts.id,
+      email: emailAccounts.email,
+      emailPassword: emailAccounts.emailPassword,
+      jmapAccountId: emailAccounts.jmapAccountId,
+      isActive: emailAccounts.isActive,
+    })
+    .from(userEmailAccounts)
+    .innerJoin(emailAccounts, eq(userEmailAccounts.emailAccountId, emailAccounts.id))
+    .where(and(eq(userEmailAccounts.userId, userId), eq(emailAccounts.isActive, true)))
     .limit(1);
 
-  if (!emailAccount) {
+  if (!emailAccountResult) {
     throw new BadRequestError('Nemáte nastavený aktívny emailový účet pre odosielanie');
   }
 
+  const emailAccount = emailAccountResult;
+
   // Build participant name
   const participantName = [
     registration.ucastnikTitul,