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/Program.cs

@@ -2,6 +2,7 @@ using System.Diagnostics.CodeAnalysis;
 using System.Text.Json.Serialization;
 using Microsoft.AspNetCore.Diagnostics;
 using Microsoft.EntityFrameworkCore;
+using Novelly.Api.Activity;
 using Novelly.Api.Agent;
 using Novelly.Api.Beats;
 using Novelly.Api.Chapters;
@@ -66,6 +67,7 @@ using (var scope = app.Services.CreateScope())
     }
 
     await ServiceUser.EnsureSeededAsync(db, builder.Configuration[ServiceApiKeyAuthenticationHandler.ConfigurationKey], app.Logger);
+    await ActivityBackfill.RunAsync(db, app.Logger);
 }
 
 app.UseSerilogRequestLogging();
@@ -119,7 +121,8 @@ app.MapNovelEndpoints()
    .MapGenreEndpoints()
    .MapOpenQuestionEndpoints()
    .MapAgentEndpoints()
-   .MapImportEndpoints();
+   .MapImportEndpoints()
+   .MapActivityEndpoints();
 
 app.Run();