refactor(jobs): modularize background job handlers

Tiku.Application/Jobs/BackgroundJobModels.cs

@@ -103,11 +103,29 @@ public sealed record BackgroundJobExecutionContext(
     string JobType,
     JsonElement Payload);
 
+public sealed record BackgroundJobHandlerResult(
+    JsonElement Result,
+    Guid? OutputAssetId = null);
+
+public sealed class BackgroundJobHandlerException(string code, string message, Exception? innerException = null)
+    : Exception(message, innerException)
+{
+    public string Code { get; } = code;
+}
+
 public interface IBackgroundJobHandler
 {
     string JobType { get; }
 
-    Task<JsonElement> HandleAsync(
+    Task<BackgroundJobHandlerResult> HandleAsync(
         BackgroundJobExecutionContext context,
         CancellationToken cancellationToken = default);
-}
+
+    Task HandleFailureAsync(
+        BackgroundJobExecutionContext context,
+        Exception exception,
+        CancellationToken cancellationToken = default)
+    {
+        return Task.CompletedTask;
+    }
+}