Update authorization policy in ConfigController

Replaced the generic [Authorize] attribute with a more specific
[Authorize(Policy = AuthPolicy.SenderOrReceiver)] to enforce
a stricter authorization policy. Added a `using` directive for
`EnvelopeGenerator.Domain.Constants` to support the new policy.

EnvelopeGenerator.API/Controllers/ConfigController.cs

@@ -1,4 +1,5 @@
 using EnvelopeGenerator.API.Models.PsPdfKitAnnotation;
+using EnvelopeGenerator.Domain.Constants;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.Extensions.Options;
@@ -13,7 +14,7 @@ namespace EnvelopeGenerator.API.Controllers;
 /// </remarks>
 [Route("api/[controller]")]
 [ApiController]
-[Authorize]
+[Authorize(Policy = AuthPolicy.SenderOrReceiver)]
 public class ConfigController(IOptionsMonitor<AnnotationParams> annotationParamsOptions) : ControllerBase
 {
     private readonly AnnotationParams _annotationParams = annotationParamsOptions.CurrentValue;