feat: implement 8 features across full stack

- Leaderboard: rank-based percentile tiers (champion/gold/silver/bronze),
    deterministic tie-breaking (xp DESC, user_id ASC)
  - Mentor chat: md: breakpoints, safe-area-inset-bottom padding
  - Learning progress: server-side progressPct, curriculum expansion safety
  - Physical orders: hard-disabled with 410 Gone
  - Certificate approval: pending/issued flow, admin issue/revoke endpoints,
    certificate_requires_approval setting (migration 0031)
  - Draft/publish: is_published on lessons and chapters, admin publish UI
    (migration 0030)
  - Video panel: static thumbnails (no preload), client-side pagination
  - Rich editor: TipTap + tiptap-markdown for lesson descriptions,
    marked + DOMPurify for SafeMarkdown rendering

backend/internal/models/models.go

@@ -107,10 +107,11 @@ func (u *User) ApplyFreezeState(now time.Time) {
 type Certificate struct {
 	ID        int64      `json:"id"`
 	UserID    int64      `json:"userId"`
-	PublicID  string     `json:"publicId"`
+	PublicID  string     `json:"publicId,omitempty"`
 	FullName  string     `json:"fullName"`
 	IssuedAt  time.Time  `json:"issuedAt"`
 	RevokedAt *time.Time `json:"revokedAt,omitempty"`
+	Status    string     `json:"status"`
 }
 
 type PhysicalOrderStatus string
@@ -164,6 +165,7 @@ type Chapter struct {
 	Description string `json:"description"`
 	Icon        string `json:"icon"`
 	SortOrder   int    `json:"sortOrder"`
+	IsPublished bool   `json:"isPublished"`
 }
 
 type Lesson struct {
@@ -180,6 +182,7 @@ type Lesson struct {
 	SortOrder              int    `json:"sortOrder"`
 	XPReward               int    `json:"xpReward"`
 	IsActive               bool   `json:"isActive"`
+	IsPublished            bool   `json:"isPublished"`
 }
 
 type MCQQuestion struct {