feat(creator): Refactor Creator flow, implement book creation pipeline & versioning, and setup Docker staging

- Relocate dashboard routing to /creator and editor workspace to /creator/edit/{BookId}
- Implement CreateBookCommand and handler with transactional default chapter seeding
- Implement PublishBookVersionCommand and GetCreatorDashboardDataQuery
- Build CreatorDashboard modal and UI components with customized dark input styles
- Add run-stage.sh script to automate staging environment setup, database migrations, and health checks
- Update developer workflow rules in GEMINI.md

src/NexusReader.Data/Persistence/AppDbContext.cs

@@ -25,6 +25,9 @@ public class AppDbContext : IdentityDbContext<NexusUser>
     public DbSet<QuizResult> QuizResults => Set<QuizResult>();
     public DbSet<SubscriptionPlan> SubscriptionPlans => Set<SubscriptionPlan>();
     public DbSet<Author> Authors => Set<Author>();
+    public DbSet<Book> Books => Set<Book>();
+    public DbSet<BookRevision> BookRevisions => Set<BookRevision>();
+    public DbSet<Chapter> Chapters => Set<Chapter>();
 
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
@@ -114,6 +117,48 @@ public class AppDbContext : IdentityDbContext<NexusUser>
             entity.HasIndex(e => e.TenantId);
         });
 
+        modelBuilder.Entity<Book>(entity =>
+        {
+            entity.HasKey(b => b.Id);
+            entity.HasIndex(b => b.TenantId);
+            entity.HasIndex(b => b.UserId);
+
+            entity.HasOne(e => e.User)
+                .WithMany()
+                .HasForeignKey(e => e.UserId)
+                .OnDelete(DeleteBehavior.Cascade);
+
+            entity.HasMany(b => b.Revisions)
+                .WithOne(r => r.Book)
+                .HasForeignKey(r => r.BookId)
+                .OnDelete(DeleteBehavior.Cascade);
+
+            entity.HasOne(b => b.CurrentDraftRevision)
+                .WithMany()
+                .HasForeignKey(b => b.CurrentDraftRevisionId)
+                .OnDelete(DeleteBehavior.Restrict);
+
+            entity.HasOne(b => b.LivePublishedRevision)
+                .WithMany()
+                .HasForeignKey(b => b.LivePublishedRevisionId)
+                .OnDelete(DeleteBehavior.Restrict);
+        });
+
+        modelBuilder.Entity<BookRevision>(entity =>
+        {
+            entity.HasKey(r => r.Id);
+
+            entity.HasMany(r => r.Chapters)
+                .WithOne(c => c.BookRevision)
+                .HasForeignKey(c => c.BookRevisionId)
+                .OnDelete(DeleteBehavior.Cascade);
+        });
+
+        modelBuilder.Entity<Chapter>(entity =>
+        {
+            entity.HasKey(c => c.Id);
+        });
+
         // Seed Subscription Plans with deterministic IDs
         modelBuilder.Entity<SubscriptionPlan>().HasData(
             new SubscriptionPlan { Id = 1, PlanName = SubscriptionPlan.FreeName, AITokenLimit = 5000, IsUnlimitedTokens = false, MonthlyPrice = 0m, StripeProductId = "prod_Free789" },