Label chapter chips by kind, not raw number

Beats, tags, locations, open questions, and character/arc-stage
responses that reference a chapter now carry a ChapterLabel/DisplayNumber
alongside the raw Number, computed via the new
ChapterDisplayNumberLookup. Front/back matter chips show their title;
body chapters show "Chapter N: Title".

src/Novelly.Api/Beats/BeatEndpoints.cs

@@ -1,3 +1,4 @@
+using Novelly.Api.Chapters;
 using Novelly.Api.Common;
 using Novelly.Api.Common.Validation;
 
@@ -45,8 +46,21 @@ public static class BeatEndpoints
             .WithSummary("Move one or more beats to another chapter, appending them to its end.");
 
         app.MapGet("/api/characters/{characterId:guid}/beats", async (
-                Guid characterId, BeatService service, CancellationToken ct) =>
-                (await service.ListForCharacterAsync(characterId, ct))?.Select(b => b.ToCharacterBeatResponse(characterId)).ToList().ToApiResult())
+                Guid characterId, BeatService service, ChapterDisplayNumberLookup chapterLabels, CancellationToken ct) =>
+            {
+                var characterBeats = await service.ListForCharacterAsync(characterId, ct);
+                if (characterBeats is null)
+                {
+                    return Results.NotFound();
+                }
+
+                var displayNumbers = characterBeats.Count > 0
+                    ? await chapterLabels.ForNovelAsync(characterBeats[0].Chapter!.NovelId, ct)
+                    : new Dictionary<Guid, int>();
+
+                return Results.Ok(characterBeats.Select(b =>
+                    b.ToCharacterBeatResponse(characterId, b.Chapter is null ? null : chapterLabels.LabelFor(b.Chapter, displayNumbers))).ToList());
+            })
             .WithTags("Beats")
             .WithSummary("Every beat this character appears in, in manuscript order.");