feat(billing): cap active users on an owner subscription

Super admin can set a seat limit of 3, 10, 20, or a negotiated number so an owner cannot add users past that cap.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/api/src/platform/dto.ts

@@ -148,6 +148,13 @@ export class ApplyBillingPlanDto {
   status?: 'active' | 'expired' | 'suspended';
 }
 
+export class SetUserCapDto {
+  @IsInt()
+  @Min(1)
+  @Max(100_000)
+  maxUsers!: number;
+}
+
 export class SetStorageQuotaDto {
   /** گیگابایت؛ null = بدون سقف */
   @IsNumber()