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/Data/NovelDbContext.cs

@@ -1,6 +1,7 @@
 using Microsoft.AspNetCore.Identity.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
 using Microsoft.EntityFrameworkCore;
+using Novelly.Api.Activity;
 using Novelly.Api.Agent;
 using Novelly.Api.Beats;
 using Novelly.Api.Chapters;
@@ -33,6 +34,7 @@ public class NovelDbContext(DbContextOptions<NovelDbContext> options) : Identity
     public DbSet<ImportJob> ImportJobs => Set<ImportJob>();
     public DbSet<Genre> Genres => Set<Genre>();
     public DbSet<NovelMember> NovelMembers => Set<NovelMember>();
+    public DbSet<ActivityEvent> ActivityEvents => Set<ActivityEvent>();
 
     Task<int> INovelDbContext.SaveChangesAsync(CancellationToken cancellationToken) => base.SaveChangesAsync(cancellationToken);
 
@@ -62,6 +64,7 @@ public interface INovelDbContext
     DbSet<Genre> Genres { get; }
     DbSet<NovellyUser> Users { get; }
     DbSet<NovelMember> NovelMembers { get; }
+    DbSet<ActivityEvent> ActivityEvents { get; }
 
     Task<int> SaveChangesAsync(CancellationToken cancellationToken = default);
 }