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>

.gitignore

@@ -3,6 +3,9 @@ go_blog
 go_blog.exe
 main.exe
 
+# fresh (live-reload) build output
+tmp/
+
 # Auto-generated runtime data (config + database + uploads)
 win/
 mac/