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/api/leaderboard_test.go

@@ -0,0 +1,31 @@
+package api
+
+import "testing"
+
+func TestTierFromRank(t *testing.T) {
+	cases := []struct {
+		rank, total int
+		want        string
+	}{
+		{1, 1, "champion"},
+		{1, 100, "champion"},
+		{1, 0, "bronze"},    // zero total → bronze (guard)
+		{0, 100, "bronze"},  // zero rank → bronze (guard)
+		{10, 100, "gold"},   // exactly 10%
+		{11, 100, "silver"}, // 11%
+		{30, 100, "silver"}, // exactly 30%
+		{31, 100, "bronze"}, // 31%
+		{5, 50, "gold"},     // 10%
+		{15, 50, "silver"},  // exactly 30%
+		{16, 50, "bronze"},  // 32% → bronze
+		{50, 100, "bronze"},
+		{2, 10, "silver"},   // 20% → silver
+		{1, 10, "champion"},
+	}
+	for _, c := range cases {
+		got := tierFromRank(c.rank, c.total)
+		if got != c.want {
+			t.Errorf("tierFromRank(%d, %d) = %q; want %q", c.rank, c.total, got, c.want)
+		}
+	}
+}