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]

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()

tests/Novelly.Api.Tests/ExceptionHandlingTests.cs

@@ -46,12 +46,6 @@ public class ExceptionHandlingTests : ServiceTestFixture
             Throws.TypeOf<ArgumentException>());
 
     [Test]
-    public async Task An_embedded_reference_to_a_missing_parent_still_throws()
-    {
-        // Creating a chapter under a nonexistent project isn't a "look this up" miss — it's
-        // an invalid precondition for the create, so it stays exceptional.
-        Assert.That(
-            async () => await Chapters.CreateAsync(Guid.NewGuid(), new CreateChapterRequest("Landfall")),
-            Throws.TypeOf<NotFoundException>());
-    }
+    public async Task Creating_a_chapter_under_a_missing_project_returns_null_rather_than_throwing() =>
+        Assert.That(await Chapters.CreateAsync(Guid.NewGuid(), new CreateChapterRequest("Landfall")), Is.Null);
 }