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

backend/internal/handler/admin/setting_handler.go

@@ -351,6 +351,10 @@ func (h *SettingHandler) GetSettings(c *gin.Context) {
 
 		AvailableChannelsEnabled: settings.AvailableChannelsEnabled,
 
+		ModelPlazaEnabled:     settings.ModelPlazaEnabled,
+		ModelPlazaRequireAuth: settings.ModelPlazaRequireAuth,
+		ModelPlazaDescription: settings.ModelPlazaDescription,
+
 		AffiliateEnabled: settings.AffiliateEnabled,
 
 		AllowUserViewErrorRequests: settings.AllowUserViewErrorRequests,

backend/internal/handler/admin/setting_handler_audit.go

@@ -528,6 +528,15 @@ func diffSettings(before *service.SystemSettings, after *service.SystemSettings,
 	if before.AvailableChannelsEnabled != after.AvailableChannelsEnabled {
 		changed = append(changed, "available_channels_enabled")
 	}
+	if before.ModelPlazaEnabled != after.ModelPlazaEnabled {
+		changed = append(changed, "model_plaza_enabled")
+	}
+	if before.ModelPlazaRequireAuth != after.ModelPlazaRequireAuth {
+		changed = append(changed, "model_plaza_require_auth")
+	}
+	if before.ModelPlazaDescription != after.ModelPlazaDescription {
+		changed = append(changed, "model_plaza_description")
+	}
 	if before.AffiliateEnabled != after.AffiliateEnabled {
 		changed = append(changed, "affiliate_enabled")
 	}

backend/internal/handler/admin/setting_handler_update.go

@@ -313,6 +313,11 @@ type UpdateSettingsRequest struct {
 	// Available Channels feature switch (user-facing)
 	AvailableChannelsEnabled *bool `json:"available_channels_enabled"`
 
+	// Model Plaza feature switches + description
+	ModelPlazaEnabled     *bool   `json:"model_plaza_enabled"`
+	ModelPlazaRequireAuth *bool   `json:"model_plaza_require_auth"`
+	ModelPlazaDescription *string `json:"model_plaza_description"`
+
 	// Affiliate (邀请返利) feature switch
 	AffiliateEnabled *bool `json:"affiliate_enabled"`
 
@@ -1678,6 +1683,24 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 			}
 			return previousSettings.AvailableChannelsEnabled
 		}(),
+		ModelPlazaEnabled: func() bool {
+			if req.ModelPlazaEnabled != nil {
+				return *req.ModelPlazaEnabled
+			}
+			return previousSettings.ModelPlazaEnabled
+		}(),
+		ModelPlazaRequireAuth: func() bool {
+			if req.ModelPlazaRequireAuth != nil {
+				return *req.ModelPlazaRequireAuth
+			}
+			return previousSettings.ModelPlazaRequireAuth
+		}(),
+		ModelPlazaDescription: func() string {
+			if req.ModelPlazaDescription != nil {
+				return *req.ModelPlazaDescription
+			}
+			return previousSettings.ModelPlazaDescription
+		}(),
 		AffiliateEnabled: func() bool {
 			if req.AffiliateEnabled != nil {
 				return *req.AffiliateEnabled
@@ -2073,6 +2096,10 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 
 		AvailableChannelsEnabled: updatedSettings.AvailableChannelsEnabled,
 
+		ModelPlazaEnabled:     updatedSettings.ModelPlazaEnabled,
+		ModelPlazaRequireAuth: updatedSettings.ModelPlazaRequireAuth,
+		ModelPlazaDescription: updatedSettings.ModelPlazaDescription,
+
 		AffiliateEnabled: updatedSettings.AffiliateEnabled,
 
 		RiskControlEnabled:          updatedSettings.RiskControlEnabled,