feat: create initial admin user + add vendor to new tenant orgs

When creating a tenant, the vendor can specify adminUsername +
adminPassword. The backend creates the user in Logto and assigns them
the owner org role. The vendor user is also auto-added to every new
org for support access.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/types/api.ts

@@ -55,6 +55,8 @@ export interface CreateTenantRequest {
   name: string;
   slug: string;
   tier?: string;
+  adminUsername?: string;
+  adminPassword?: string;
 }
 
 // Tenant portal API types