diff --git a/apps/api/src/app.ts b/apps/api/src/app.ts
index 3a8b56e..20a7a00 100644
--- a/apps/api/src/app.ts
+++ b/apps/api/src/app.ts
@@ -45,6 +45,10 @@ import metricasRoutes from './routes/metricas.routes.js';
 
 const app: Express = express();
 
+// Trust proxy — la app corre detrás de Cloudflare/nginx. Necesario para que
+// express-rate-limit lea correctamente X-Forwarded-For sin lanzar warnings.
+app.set('trust proxy', 1);
+
 // Security. Helmet default incluye un CSP restrictivo que puede chocar con el
 // frontend cuando éste embebe recursos propios (ej: /terminos embebe el PDF de
 // /legal/). Dejamos CSP off en el API y centralizamos los headers de seguridad
diff --git a/apps/api/src/services/email/email.service.ts b/apps/api/src/services/email/email.service.ts
index 98d276c..517d890 100644
--- a/apps/api/src/services/email/email.service.ts
+++ b/apps/api/src/services/email/email.service.ts
@@ -18,7 +18,7 @@ async function sendEmail(to: string, subject: string, html: string) {
 }
 
 export const emailService = {
-  sendWelcome: async (to: string, data: { nombre: string; email: string; tempPassword: string }) => {
+  sendWelcome: async (to: string, data: { nombre: string; email: string; tempPassword: string | null | undefined }) => {
     const { welcomeEmail } = await import('./templates/welcome.js');
     await sendEmail(to, 'Bienvenido a Horux360', welcomeEmail(data));
   },
@@ -60,7 +60,7 @@ export const emailService = {
     clienteRfc: string;
     adminEmail: string;
     adminNombre: string;
-    tempPassword: string;
+    tempPassword: string | null | undefined;
     databaseName: string;
     plan: string;
   }) => {
diff --git a/apps/api/src/services/email/templates/new-client-admin.ts b/apps/api/src/services/email/templates/new-client-admin.ts
index d9894f8..7941ce1 100644
--- a/apps/api/src/services/email/templates/new-client-admin.ts
+++ b/apps/api/src/services/email/templates/new-client-admin.ts
@@ -25,7 +25,7 @@ export function newClientAdminEmail(data: {
   clienteRfc: string;
   adminEmail: string;
   adminNombre: string;
-  tempPassword: string;
+  tempPassword: string | null | undefined;
   databaseName: string;
   plan: string;
 }): string {
@@ -46,7 +46,7 @@ export function newClientAdminEmail(data: {
       ${sectionHeader('Credenciales del usuario', C.secondary)}
       ${row('Nombre', escapeHtml(data.adminNombre))}
       ${row('Email', `<span style="font-family:monospace;">${escapeHtml(data.adminEmail)}</span>`)}
-      ${row('Contraseña temporal', `<code style="background-color:${C.bgLight};padding:4px 10px;border-radius:6px;font-size:13px;color:#dc2626;border:1px solid ${C.border};">${escapeHtml(data.tempPassword)}</code>`, true)}
+      ${row('Contraseña temporal', `<code style="background-color:${C.bgLight};padding:4px 10px;border-radius:6px;font-size:13px;color:#dc2626;border:1px solid ${C.border};">${escapeHtml(data.tempPassword || 'N/A - usuario ya existía')}</code>`, true)}
     </table>
 
     <div style="background-color:#fef2f2;border-left:4px solid #ef4444;border-radius:8px;padding:12px 16px;margin:0 0 16px;">
diff --git a/apps/api/src/services/email/templates/welcome.ts b/apps/api/src/services/email/templates/welcome.ts
index 279025f..0e717ab 100644
--- a/apps/api/src/services/email/templates/welcome.ts
+++ b/apps/api/src/services/email/templates/welcome.ts
@@ -1,6 +1,6 @@
 import { baseTemplate, heading, primaryButton, infoBox, BRAND_COLORS as C } from './base.js';
 
-export function welcomeEmail(data: { nombre: string; email: string; tempPassword: string }): string {
+export function welcomeEmail(data: { nombre: string; email: string; tempPassword: string | null | undefined }): string {
   return baseTemplate(`
     ${heading('Bienvenido a Horux 360')}
     <p style="color:${C.textPrimary};margin:0 0 16px;">Hola <strong>${data.nombre}</strong>,</p>
diff --git a/apps/api/src/services/tenants.service.ts b/apps/api/src/services/tenants.service.ts
index 25ed8d7..256ce5a 100644
--- a/apps/api/src/services/tenants.service.ts
+++ b/apps/api/src/services/tenants.service.ts
@@ -91,26 +91,41 @@ export async function createTenant(data: {
     }
   });
 
-  // 3. Create admin user with temp password
-  const tempPassword = randomBytes(4).toString('hex'); // 8-char random
-  const hashedPassword = await bcrypt.hash(tempPassword, 10);
-
-  // Get owner role ID from roles table (rol que asignamos al dueño del tenant al crearlo)
+  // 3. Create or reuse admin user
   const ownerRol = await prisma.rol.findUnique({ where: { nombre: 'owner' } });
   if (!ownerRol) throw new Error('Rol owner no encontrado en la base de datos');
 
-  const user = await prisma.user.create({
-    data: {
-      email: data.adminEmail,
-      passwordHash: hashedPassword,
-      nombre: data.adminNombre,
-      lastTenantId: tenant.id,
-    },
-  });
+  let user = await prisma.user.findUnique({ where: { email: data.adminEmail } });
+  let tempPassword: string | null = null;
 
-  // Crea membership owner del nuevo user en su tenant (fase 4 multi-tenant)
-  await prisma.tenantMembership.create({
-    data: {
+  if (!user) {
+    tempPassword = randomBytes(4).toString('hex'); // 8-char random
+    const hashedPassword = await bcrypt.hash(tempPassword, 10);
+    user = await prisma.user.create({
+      data: {
+        email: data.adminEmail,
+        passwordHash: hashedPassword,
+        nombre: data.adminNombre,
+        lastTenantId: tenant.id,
+      },
+    });
+  } else {
+    // User ya existe: actualizar lastTenantId y nombre si cambió
+    await prisma.user.update({
+      where: { id: user.id },
+      data: {
+        lastTenantId: tenant.id,
+        ...(data.adminNombre && data.adminNombre !== user.nombre ? { nombre: data.adminNombre } : {}),
+      },
+    });
+  }
+
+  // Crea membership owner del user en su tenant (fase 4 multi-tenant).
+  // Si ya existía (re-invite a otro tenant), reactivar.
+  await prisma.tenantMembership.upsert({
+    where: { userId_tenantId: { userId: user.id, tenantId: tenant.id } },
+    update: { rolId: ownerRol.id, isOwner: true, active: true },
+    create: {
       userId: user.id,
       tenantId: tenant.id,
       rolId: ownerRol.id,