feat(backend): leaderboard tiers, N+1 fix, contributors, certificate approval, physical disable

- leaderboard: batch-fetch users (GetUsersByIDs), deterministic tie-break by user_id
- leaderboard: tier system (champion/gold/silver/bronze) based on rank percentile
- store: HasCompletedCurriculum requires cert.Status == "issued"; pending does not satisfy
- certificate: issueCertificateIfEligible creates pending row; admin approves via handleAdminIssueCertificate
- certificate: handleRequestPhysicalCertificate returns 410 Gone (feature disabled)
- settings: contributors CRUD (loadContributors, validateContributors, handleAdminContributorAvatarUpload)
- community: expose contributors in /api/community response
- rate_limit: separate DevMode bypass from redis-nil check in allowWSHandshake
- router: remove /api/search route; fix missing tab indent on /api/meta line
- migration 0032: contributors app_setting with seed entry

backend/internal/api/community.go

@@ -5,7 +5,9 @@ import (
 )
 
 func (s *Server) handleCommunity(w http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
 	writeJSON(w, http.StatusOK, map[string]any{
-		"socials": s.loadSocials(r.Context()),
+		"socials":      s.loadSocials(ctx),
+		"contributors": s.loadContributors(ctx),
 	})
 }