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/Chapters/ChapterEndpoints.cs

@@ -18,7 +18,13 @@ public static class ChapterEndpoints
         projectScoped.MapPost("/", async (
                 Guid projectId, CreateChapterRequest request, ChapterService service, CancellationToken ct) =>
             {
-                var created = (await service.CreateAsync(projectId, request, ct)).ToResponse();
+                var chapter = await service.CreateAsync(projectId, request, ct);
+                if (chapter is null)
+                {
+                    return Results.NotFound();
+                }
+
+                var created = chapter.ToResponse();
                 return Results.Created($"/api/chapters/{created.Id}", created);
             })
             .WithSummary("Add a chapter.");

src/Novelly.Api/Chapters/ChapterService.cs

@@ -2,7 +2,6 @@ using Microsoft.EntityFrameworkCore;
 using Novelly.Api.Common;
 using Novelly.Api.Common.Validation;
 using Novelly.Api.Data;
-using Novelly.Api.Projects;
 using Novelly.Api.Tags;
 
 namespace Novelly.Api.Chapters;
@@ -39,7 +38,8 @@ public class ChapterService(
         return await FindAsync(id, ct);
     }
 
-    public async Task<Chapter> CreateAsync(Guid projectId, CreateChapterRequest request, CancellationToken ct = default)
+    /// <summary>Null when no project has this id — a lookup miss is expected, not exceptional.</summary>
+    public async Task<Chapter?> CreateAsync(Guid projectId, CreateChapterRequest request, CancellationToken ct = default)
     {
         Guard.Default(projectId, nameof(projectId));
         Guard.Null(request, nameof(request));
@@ -49,8 +49,8 @@ public class ChapterService(
 
         if (!await db.Projects.AnyAsync(p => p.Id == projectId, ct))
         {
-            logger.LogWarning("Rejected chapter creation: project {ProjectId} not found", projectId);
-            throw new NotFoundException(nameof(Project), projectId);
+            logger.LogInformation("Rejected chapter creation: project {ProjectId} not found", projectId);
+            return null;
         }
 
         var chapter = new Chapter