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/CharacterArcTests.cs

@@ -183,11 +183,10 @@ public class CharacterArcTests : ServiceTestFixture
     }
 
     [Test]
-    public void Reordering_with_an_unknown_stage_is_refused() =>
+    public async Task Reordering_with_an_unknown_stage_returns_null_rather_than_throwing() =>
         Assert.That(
-            async () => await Arcs.ReorderAsync(
-                _characterId, new ReorderArcStagesRequest([Guid.NewGuid()])),
-            Throws.TypeOf<NotFoundException>());
+            await Arcs.ReorderAsync(_characterId, new ReorderArcStagesRequest([Guid.NewGuid()])),
+            Is.Null);
 
     [Test]
     public async Task The_character_page_sees_every_beat_they_appear_in_across_the_book()