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/Beats/BeatEndpoints.cs

@@ -18,14 +18,20 @@ public static class BeatEndpoints
         chapterScoped.MapPost("/", async (
                 Guid chapterId, CreateBeatRequest request, BeatService service, CancellationToken ct) =>
             {
-                var created = (await service.CreateAsync(chapterId, request, ct)).ToResponse();
+                var beat = await service.CreateAsync(chapterId, request, ct);
+                if (beat is null)
+                {
+                    return Results.NotFound();
+                }
+
+                var created = beat.ToResponse();
                 return Results.Created($"/api/beats/{created.Id}", created);
             })
             .WithSummary("Add a beat to a chapter's outline.");
 
         chapterScoped.MapPost("/reorder", async (
                 Guid chapterId, ReorderBeatsRequest request, BeatService service, CancellationToken ct) =>
-                Results.Ok((await service.ReorderAsync(chapterId, request, ct)).Select(b => b.ToResponse())))
+                (await service.ReorderAsync(chapterId, request, ct))?.Select(b => b.ToResponse()).ToList().ToApiResult())
             .WithSummary("Renumber a chapter's beats to match the order given.");
 
         app.MapGet("/api/characters/{characterId:guid}/beats", async (