Remove NotFoundException; services return null on lookup miss

A missing record isn't exceptional — services now return null (logged
at Info) instead of throwing, and endpoints map null to 404. Agent and
import toolsets route not-found through their existing OrNotFound
result pattern rather than a caught exception.

src/Novelly.Api/Chapters/ChapterEndpoints.cs

@@ -18,7 +18,13 @@ public static class ChapterEndpoints
         projectScoped.MapPost("/", async (
                 Guid projectId, CreateChapterRequest request, ChapterService service, CancellationToken ct) =>
             {
-                var created = (await service.CreateAsync(projectId, request, ct)).ToResponse();
+                var chapter = await service.CreateAsync(projectId, request, ct);
+                if (chapter is null)
+                {
+                    return Results.NotFound();
+                }
+
+                var created = chapter.ToResponse();
                 return Results.Created($"/api/chapters/{created.Id}", created);
             })
             .WithSummary("Add a chapter.");