fix: E-Mail-Formatvalidierung zu ReceiverCreateDto hinzugefügt und Modellvalidierung im Create-Methoden behandelt

- `[EmailAddress]`-Attribut auf `EmailAddress` in `ReceiverCreateDto` angewendet, um das E-Mail-Format zu validieren.
- `ModelState.IsValid`-Überprüfung zur `Create`-Methode hinzugefügt, um Validierungsfehler vor dem Aufruf der Basis-`Create`-Methode zu behandeln.

EnvelopeGenerator.GeneratorAPI/Controllers/ReceiverController.cs

@@ -4,7 +4,6 @@ using EnvelopeGenerator.Application.Contracts;
 using EnvelopeGenerator.Application.DTOs.Receiver;
 using EnvelopeGenerator.Domain.Entities;
 using Microsoft.AspNetCore.Mvc;
-using System.ComponentModel.DataAnnotations.Schema;
 
 namespace EnvelopeGenerator.GeneratorAPI.Controllers
 {
@@ -12,7 +11,7 @@ namespace EnvelopeGenerator.GeneratorAPI.Controllers
     [ApiController]
     public class ReceiverController : CRUDControllerBaseWithErrorHandling<IReceiverService, ReceiverCreateDto, ReceiverReadDto, ReceiverUpdateDto, Receiver, int>
     {
-        public ReceiverController(ILogger logger, IReceiverService service) : base(logger, service)
+        public ReceiverController(ILogger<ReceiverController> logger, IReceiverService service) : base(logger, service)
         {
         }
 
@@ -36,5 +35,14 @@ namespace EnvelopeGenerator.GeneratorAPI.Controllers
                         return StatusCode(StatusCodes.Status500InternalServerError);
                     });
         }
+
+        [HttpPost]
+        public async override Task<IActionResult> Create(ReceiverCreateDto createDto)
+        {
+            if (!ModelState.IsValid)
+                return BadRequest(ModelState);
+
+            return await base.Create(createDto);
+        }
     }
 }