feat(invitations): flujo de invitacion de clientes por email

Backend:
- Nuevo modelo Prisma ClientInvitation con token unico, expiracion
  y estados (pending/accepted/expired).
- Migracion: 20260511213955_add_client_invitations
- Service client-invitations.service.ts: crear invitacion,
  validar token, registrar desde invitacion (reutiliza logica
  de creacion de tenant + usuario de despacho.service).
- Controller + routes: POST /invitations/client (admin),
  GET /invitations/client/validate/:token (publico),
  POST /invitations/client/register/:token (publico),
  GET /invitations/client (admin).
- Email template client-invitation.ts con link a
  /invitacion/registro/{token}.
- Agregado sendClientInvitation a email.service.

Frontend:
- Pagina /invitacion/registro/[token] para que el invitado
  complete registro (nombre, password, despacho, RFC, perfil).
- Pagina /admin/invitar-cliente para que admin global envie
  invitaciones y vea el historial.
- Hooks useCreateInvitation, useValidateInvitationToken,
  useRegisterFromInvitation, useClientInvitations.
- API client lib/api/client-invitations.ts.

Infra:
- PM2 ecosystem.config.js: usa node --import tsx con
  kill_timeout aumentado a 15s para evitar EADDRINUSE.
- React Query retry=2 con delay exponencial para resiliencia.

Refs: docs/CAMBIOS-2026-05-09.md

apps/api/prisma/migrations/20260511213955_add_client_invitations/migration.sql

@@ -0,0 +1,28 @@
+-- CreateTable
+CREATE TABLE "client_invitations" (
+    "id" TEXT NOT NULL,
+    "email" TEXT NOT NULL,
+    "invited_by" TEXT NOT NULL,
+    "nombre_despacho" TEXT,
+    "rfc" TEXT,
+    "status" TEXT NOT NULL DEFAULT 'pending',
+    "token" TEXT NOT NULL,
+    "sent_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "expires_at" TIMESTAMP(3) NOT NULL,
+    "accepted_at" TIMESTAMP(3),
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "client_invitations_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "client_invitations_token_key" ON "client_invitations"("token");
+
+-- CreateIndex
+CREATE INDEX "client_invitations_token_idx" ON "client_invitations"("token");
+
+-- CreateIndex
+CREATE INDEX "client_invitations_status_idx" ON "client_invitations"("status");
+
+-- CreateIndex
+CREATE INDEX "client_invitations_email_idx" ON "client_invitations"("email");