Add ChapterKind for front/back matter chapters

Chapters can now be marked FrontMatter/Body/BackMatter. Number stays
the manuscript sort key for every chapter; the author-facing display
number is now computed per-request as the chapter's ordinal among
Body chapters only, so a foreword or afterword no longer shifts the
numbering of the rest of the book. Surfaced through the API, agent
toolset, and import toolset.

src/Novelly.Api/Chapters/ChapterService.cs

@@ -73,6 +73,7 @@ public class ChapterService(
             NovelId = novelId,
             Title = request.Title,
             Number = request.Number ?? await NextChapterNumberAsync(novelId, ct),
+            Kind = request.Kind,
             Summary = request.Summary,
             Notes = request.Notes,
             Status = request.Status,
@@ -116,6 +117,7 @@ public class ChapterService(
 
         chapter.Title = Patch.Apply(chapter.Title, request.Title) ?? chapter.Title;
         chapter.Number = request.Number ?? chapter.Number;
+        chapter.Kind = request.Kind ?? chapter.Kind;
         chapter.Summary = Patch.Apply(chapter.Summary, request.Summary);
         chapter.Notes = Patch.Apply(chapter.Notes, request.Notes);
         chapter.Status = request.Status ?? chapter.Status;
@@ -166,6 +168,15 @@ public class ChapterService(
         return true;
     }
 
+    public async Task<int?> DisplayNumberAsync(Chapter chapter, CancellationToken ct = default)
+    {
+        if (chapter.Kind != ChapterKind.Body)
+            return null;
+
+        return await db.Chapters.CountAsync(
+            c => c.NovelId == chapter.NovelId && c.Kind == ChapterKind.Body && c.Number <= chapter.Number, ct);
+    }
+
     private async Task<int> NextChapterNumberAsync(Guid novelId, CancellationToken ct)
     {
         logger.LogDebug("Computing next chapter number for novel {NovelId}", novelId);