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/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);
 }