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/db/migrations/0032_contributors.sql

@@ -0,0 +1,6 @@
+-- Add contributors setting key. Seed one verified entry (sole git author).
+-- Admins can update via the site settings panel.
+INSERT INTO app_settings (key, value) VALUES (
+  'contributors',
+  '[{"name":"Amir Husayn Panahifar","role":"بنیان‌گذار و توسعه‌دهنده","bio":"","url":"","avatar":"","contributions":["code"]}]'
+) ON CONFLICT (key) DO NOTHING;