feat: add article comment system with moderation

- Comment model with nested replies (ParentID), dual authorship
  (logged-in UserID / anonymous GuestToken cookie), email hash for
  Gravatar, private flag, and moderation status
- CommentConfig singleton (enabled / allow guest / guest-require-approval
  / use Gravatar) cached like the other platform config
- Markdown comments with built-in emoji picker, preview, and markdown
  help; rendered client-side via marked + DOMPurify, with server-side
  HTML/dangerous-scheme stripping as a first XSS defense
- Private comments visible only to admin and the author; pending
  comments visible only to admin and the author
- Admin moderation list (pending/approved/rejected/all tabs) with
  approve/reject/delete and a pending-count badge
- Comment settings page with the four toggles
- One-time session flash notice (auto-dismissed after 4s) so the
  "comment posted" banner no longer persists across refreshes

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

models/db.go

@@ -45,7 +45,7 @@ func InitDB(cfg *config.Config) *gorm.DB {
 	}
 
 	// Auto-migrate tables (idempotent).
-	if err := db.AutoMigrate(&User{}, &Article{}, &SiteSetting{}, &UploadConfig{}, &UploadFileType{}, &DownloadBaseURL{}, &Attachment{}); err != nil {
+	if err := db.AutoMigrate(&User{}, &Article{}, &SiteSetting{}, &UploadConfig{}, &UploadFileType{}, &DownloadBaseURL{}, &Attachment{}, &Comment{}, &CommentConfig{}); err != nil {
 		log.Fatalf("Failed to auto-migrate database: %v", err)
 	}
 
@@ -53,6 +53,7 @@ func InitDB(cfg *config.Config) *gorm.DB {
 	seedSiteSettings(db)
 	seedUploadConfig(db)
 	seedUploadFileTypes(db)
+	seedCommentConfig(db)
 
 	// First-run seed: create admin user if no users exist.
 	var count int64