feat: add model plaza with group-scoped pricing showcase

- public /model-plaza page (standalone + admin-embedded) listing groups
  with discounted effective prices alongside LiteLLM official reference
- faceted platform/group/rate filters: cross-dimension options gray out
  instead of disappearing, platform-tinted chips via accent color-mix
- paid-price columns highlighted with per-platform tint band
- OptionalJWT middleware so anonymous and signed-in users share one route
- admin settings: enable switch, require-auth switch, markdown description

frontend/src/types/index.ts

@@ -239,6 +239,8 @@ export interface PublicSettings {
   channel_monitor_enabled: boolean
   channel_monitor_default_interval_seconds: number
   available_channels_enabled: boolean
+  model_plaza_enabled: boolean
+  model_plaza_require_auth: boolean
   service_quota_enabled: boolean
   affiliate_enabled: boolean
   allow_user_view_error_requests?: boolean