Add GitHub-style activity contribution graph

Record create/update/delete events across novel content (chapters, beats,
characters, arc stages, tags, locations, questions) into an append-only
ActivityEvent log, aggregate by UTC day, and surface as a heatmap on the
novels list and each novel's dashboard. Backfills history from existing
CreatedAt timestamps on first boot after the migration.

src/Novelly.Api/Chapters/ChapterService.cs

@@ -1,4 +1,5 @@
 using Microsoft.EntityFrameworkCore;
+using Novelly.Api.Activity;
 using Novelly.Api.Common;
 using Novelly.Api.Common.Validation;
 using Novelly.Api.Data;
@@ -13,6 +14,7 @@ public class ChapterService(
     NovelAccessService access,
     TagService tags,
     LocationService locations,
+    ActivityLog activity,
     ILogger<ChapterService> logger,
     IModelValidator<CreateChapterRequest> createValidator,
     IModelValidator<UpdateChapterRequest> updateValidator)
@@ -90,6 +92,7 @@ public class ChapterService(
         }
 
         db.Chapters.Add(chapter);
+        activity.Record(novelId, ActivityEntityKind.Chapter, ActivityAction.Created, chapter.Id, chapter.WordCount);
         await db.SaveChangesAsync(ct);
 
         return (await FindAsync(chapter.Id, ct))!;
@@ -118,6 +121,8 @@ public class ChapterService(
         chapter.Status = request.Status ?? chapter.Status;
         chapter.TargetWordCount = request.TargetWordCount ?? chapter.TargetWordCount;
 
+        var wordCountBeforeEdit = chapter.WordCount;
+
         if (request.Prose is not null)
         {
             chapter.Prose = Patch.Apply(chapter.Prose, request.Prose);
@@ -136,6 +141,7 @@ public class ChapterService(
             chapter.Locations = await locations.ResolveAsync(chapter.NovelId, locationNames, ct);
         }
 
+        activity.Record(chapter.NovelId, ActivityEntityKind.Chapter, ActivityAction.Updated, chapter.Id, chapter.WordCount - wordCountBeforeEdit);
         await db.SaveChangesAsync(ct);
         return (await FindAsync(id, ct))!;
     }
@@ -155,6 +161,7 @@ public class ChapterService(
         await access.RequireAsync(chapter.NovelId, NovelPermission.DeleteContent, ct);
 
         db.Chapters.Remove(chapter);
+        activity.Record(chapter.NovelId, ActivityEntityKind.Chapter, ActivityAction.Deleted, chapter.Id, -chapter.WordCount);
         await db.SaveChangesAsync(ct);
         return true;
     }