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/ChapterContracts.cs

@@ -10,6 +10,8 @@ public record ChapterSummaryResponse(
     Guid Id,
     Guid NovelId,
     int Number,
+    ChapterKind Kind,
+    int? DisplayNumber,
     string Title,
     string? Summary,
     IReadOnlyList<LocationResponse> Locations,
@@ -24,6 +26,8 @@ public record ChapterResponse(
     Guid Id,
     Guid NovelId,
     int Number,
+    ChapterKind Kind,
+    int? DisplayNumber,
     string Title,
     string? Summary,
     IReadOnlyList<LocationResponse> Locations,
@@ -39,6 +43,7 @@ public record ChapterResponse(
 public record CreateChapterRequest(
     string Title,
     int? Number = null,
+    ChapterKind Kind = ChapterKind.Body,
     string? Summary = null,
     IReadOnlyList<string>? Locations = null,
     string? Notes = null,
@@ -63,6 +68,7 @@ public class CreateChapterRequestValidator : IModelValidator<CreateChapterReques
 public record UpdateChapterRequest(
     string? Title = null,
     int? Number = null,
+    ChapterKind? Kind = null,
     string? Summary = null,
     IReadOnlyList<string>? Locations = null,
     string? Notes = null,
@@ -115,8 +121,8 @@ file static class ChapterValidation
 
 public static class ChapterMapping
 {
-    public static ChapterResponse ToResponse(this Chapter c) => new(
-        c.Id, c.NovelId, c.Number, c.Title, c.Summary,
+    public static ChapterResponse ToResponse(this Chapter c, int? displayNumber = null) => new(
+        c.Id, c.NovelId, c.Number, c.Kind, displayNumber, c.Title, c.Summary,
         [.. c.Locations.OrderBy(l => l.Name).Select(l => l.ToResponse())],
         c.Notes,
         c.Status, c.TargetWordCount,
@@ -125,8 +131,8 @@ public static class ChapterMapping
         [.. c.Tags.OrderBy(t => t.Name).Select(t => t.ToResponse())],
         c.UpdatedAt);
 
-    public static ChapterSummaryResponse ToSummaryResponse(this Chapter c) => new(
-        c.Id, c.NovelId, c.Number, c.Title, c.Summary,
+    public static ChapterSummaryResponse ToSummaryResponse(this Chapter c, int? displayNumber = null) => new(
+        c.Id, c.NovelId, c.Number, c.Kind, displayNumber, c.Title, c.Summary,
         [.. c.Locations.OrderBy(l => l.Name).Select(l => l.ToResponse())],
         c.Status, c.TargetWordCount,
         c.Beats.Count, c.WordCount,