Add signup trial que crea tenant y owner

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mvp/b2c/src/db/auth-queries.ts

@@ -45,3 +45,8 @@ export async function createTenantWithOwner(input: {
 
   return { tenant, user };
 }
+
+export async function slugDisponible(slug: string): Promise<boolean> {
+  const [row] = await db.select({ id: tenants.id }).from(tenants).where(eq(tenants.slug, slug)).limit(1);
+  return !row;
+}