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/comment_config.go

@@ -0,0 +1,31 @@
+package models
+
+import "time"
+
+// CommentConfig holds the singleton (id=1) global comment policy.
+type CommentConfig struct {
+	ID                   uint      `gorm:"primarykey" json:"id"`
+	Enabled              bool      `gorm:"default:true" json:"enabled"`                // master switch for the comment system
+	AllowGuest           bool      `gorm:"default:true" json:"allow_guest"`             // whether anonymous (non-logged-in) comments are allowed
+	GuestRequireApproval bool      `gorm:"default:false" json:"guest_require_approval"` // hold guest comments in the moderation queue
+	UseGravatar          bool      `gorm:"default:true" json:"use_gravatar"`            // when false, avatars render as a text-initial placeholder
+	UpdatedBy            uint      `gorm:"index" json:"updated_by"`
+	UpdatedAt            time.Time `json:"updated_at"`
+}
+
+// TableName overrides the default GORM table name.
+func (CommentConfig) TableName() string {
+	return "comment_configs"
+}
+
+// defaultCommentConfig returns the in-memory fallback used before the DB row is
+// seeded, matching the seed defaults.
+func defaultCommentConfig() *CommentConfig {
+	return &CommentConfig{
+		ID:                   1,
+		Enabled:              true,
+		AllowGuest:           true,
+		GuestRequireApproval: false,
+		UseGravatar:          true,
+	}
+}