fix(identity): internal render-charge request binding (record → class)

The nested positional record ChargeReq(Guid UserId) failed System.Text.Json
binding under the snake_case policy (400). Use a plain class with a settable
property. Verified: consume decrements + blocks at 0, refund restores, bad
service token → 401.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/identity/FlatRender.IdentitySvc/Controllers/InternalController.cs

@@ -11,7 +11,7 @@ namespace FlatRender.IdentitySvc.Controllers;
 [Route("v1/internal")]
 public class InternalController(AdminService svc, IConfiguration config) : ControllerBase
 {
-    public record ChargeReq(Guid UserId);
+    public class ChargeReq { public Guid UserId { get; set; } }
 
     private bool ServiceTokenValid()
     {