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/Chapter.cs

@@ -16,6 +16,8 @@ public class Chapter
 
     public int Number { get; set; }
 
+    public ChapterKind Kind { get; set; } = ChapterKind.Body;
+
     public string Title { get; set; } = string.Empty;
 
     public string? Summary { get; set; }
@@ -44,6 +46,7 @@ public class ChapterEntityTypeConfiguration : IEntityTypeConfiguration<Chapter>
     {
         entity.Property(c => c.Title).IsRequired().HasMaxLength(300);
         entity.Property(c => c.Status).HasConversion<string>().HasMaxLength(32);
+        entity.Property(c => c.Kind).HasConversion<string>().HasMaxLength(32);
         entity.HasIndex(c => new { c.NovelId, c.Number });
     }
 }