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.

tests/Novelly.Api.Tests/BeatServiceTests.cs

@@ -67,13 +67,11 @@ public class BeatServiceTests : ServiceTestFixture
     }
 
     [Test]
-    public void Reordering_with_an_unknown_beat_is_refused()
+    public async Task Reordering_with_an_unknown_beat_returns_null_rather_than_throwing()
     {
-        Beats.CreateAsync(_chapterId, new CreateBeatRequest("First")).Wait();
+        await Beats.CreateAsync(_chapterId, new CreateBeatRequest("First"));
 
-        Assert.That(
-            async () => await Beats.ReorderAsync(_chapterId, new ReorderBeatsRequest([Guid.NewGuid()])),
-            Throws.TypeOf<NotFoundException>());
+        Assert.That(await Beats.ReorderAsync(_chapterId, new ReorderBeatsRequest([Guid.NewGuid()])), Is.Null);
     }
 
     [Test]