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/api/admin/settings.ts

@@ -659,6 +659,11 @@ export interface SystemSettings {
   // Available Channels feature switch
   available_channels_enabled: boolean;
 
+  // Model Plaza feature switches + description
+  model_plaza_enabled: boolean;
+  model_plaza_require_auth: boolean;
+  model_plaza_description: string;
+
   // Affiliate (邀请返利) feature switch
   affiliate_enabled: boolean;
 
@@ -930,6 +935,11 @@ export interface UpdateSettingsRequest {
   // Available Channels feature switch
   available_channels_enabled?: boolean;
 
+  // Model Plaza feature switches + description
+  model_plaza_enabled?: boolean;
+  model_plaza_require_auth?: boolean;
+  model_plaza_description?: string;
+
   // Affiliate (邀请返利) feature switch
   affiliate_enabled?: boolean;