Implement Plunk email sending service with HttpClient integration and DTO definition.

API/Services/PlunkEmailSender.cs

@@ -1,11 +1,36 @@
-using Microsoft.AspNetCore.Identity.UI.Services;
+using System.Net.Http.Headers;
+using System.Text;
+using System.Text.Json;
+using API.Models.Outgoing;
+using Microsoft.AspNetCore.Identity.UI.Services;
 
 namespace API.Services;
 
-public class PlunkEmailSender : IEmailSender
+public class PlunkEmailSender(HttpClient httpClient, string plunkSecretKey) : IEmailSender
 {
-    public Task SendEmailAsync(string email, string subject, string htmlMessage)
+    public async Task SendEmailAsync(string email, string subject, string htmlMessage)
     {
-        throw new NotImplementedException();
+        var requestBody = new PlunkEmailDto()
+        {
+            To = email,
+            Subject = subject,
+            Body = htmlMessage
+        };
+
+        var jsonContent = new StringContent(
+            JsonSerializer.Serialize(requestBody),
+            Encoding.UTF8,
+            "application/json"
+        );
+
+        httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", plunkSecretKey);
+
+        var response = await httpClient.PostAsync("https://api.useplunk.com/v1/send", jsonContent);
+
+        if (!response.IsSuccessStatusCode)
+        {
+            var error = await response.Content.ReadAsStringAsync();
+            throw new Exception($"Fehler beim Senden der E-Mail über Plunk: {error}");
+        }
     }
 }