feat(settings): 新增「站点类型」三态开关，关闭订阅后用户端收起所有「订阅」入口与文案

后台功能开关页新增「站点类型」单选：充值 & 订阅（默认，即现状）/ 仅充值 / 仅订阅。
它由两个后端开关派生并在保存时同时写回：
- subscription_enabled（新增；默认 true，缺省/空值视为开启，仅显式 false 关闭），按既有
  开关模式贯通常量、公共设置、SSR 注入载荷、默认值、更新写入、DTO、审计 diff 与前端注册表；
- payment_balance_disabled（支付配置里既有的 BALANCE_PAYMENT_DISABLED，此前只能走接口/数据库
  改），补进公共设置与注入载荷，让用户端在任何鉴权请求之前就能推导站点类型。
两者都关闭是接口层可表达但 UI 不提供的组合，前端归为「仅充值」，后台保存时会把余额充值重新打开。

「仅充值」时的表现：
- 用户侧边栏隐藏「我的订阅」，「充值/订阅」入口收窄为「充值」；管理端侧边栏隐藏「订阅管理」
  入口（页面仍可通过地址访问）；
- 购买页不再渲染「订阅」tab，只剩单个 tab 时顶部切换器随之隐藏；tab 列表变化时当前 tab
  自动收敛到可用项（关闭订阅回到充值、仅订阅站点重新打开订阅进入订阅），列表为空时展示
  「充值与订阅均暂未开放」而不是一个必定被后端拒绝的充值表单；
- /purchase 的页头标题/描述与 document.title 改走 resolveRouteMetaKeys 按站点类型切换；
- /subscriptions 路由新增 requiresSubscription 守卫，直达时重定向到仪表盘；
- 顶栏订阅进度徽章不再挂载，App.vue、PaymentView 与徽章组件都不再预加载/轮询订阅接口；
- 用量页隐藏「计费类型（余额/订阅）」筛选，Key 用量公开页「订阅类型」行改为「计费方式」。
「仅订阅」时侧边栏入口与页头显示为「订阅」，购买页只剩订阅 tab（余额下单由后端既有逻辑拒绝）。

购买订阅接口、已有订阅的计费、兑换码发放订阅与管理端订阅管理页面本身均不受影响。
featureFlags 抽出纯函数 resolveFeatureFlag，新增 utils/siteBillingMode 承载三态派生与反解，
PaymentView 测试改用响应式 store mock 覆盖开关双向切换。

backend/internal/handler/admin/setting_handler.go

@@ -383,6 +383,7 @@ func (h *SettingHandler) GetSettings(c *gin.Context) {
 		GrokDefaultBaseURLMode:         settings.GrokDefaultBaseURLMode,
 
 		AvailableChannelsEnabled: settings.AvailableChannelsEnabled,
+		SubscriptionEnabled:      settings.SubscriptionEnabled,
 
 		ModelPlazaEnabled:       settings.ModelPlazaEnabled,
 		ModelPlazaRequireAuth:   settings.ModelPlazaRequireAuth,

backend/internal/handler/admin/setting_handler_audit.go

@@ -579,6 +579,9 @@ func diffSettings(before *service.SystemSettings, after *service.SystemSettings,
 	if before.AvailableChannelsEnabled != after.AvailableChannelsEnabled {
 		changed = append(changed, "available_channels_enabled")
 	}
+	if before.SubscriptionEnabled != after.SubscriptionEnabled {
+		changed = append(changed, "subscription_enabled")
+	}
 	if before.ModelPlazaEnabled != after.ModelPlazaEnabled {
 		changed = append(changed, "model_plaza_enabled")
 	}

backend/internal/handler/admin/setting_handler_partial_payload_test.go

@@ -186,3 +186,20 @@ func TestUpdateSettingsValidatesTencentCaptchaAppIDWhenEnabledFlagIsOmitted(t *t
 	require.Equal(t, http.StatusBadRequest, rec.Code)
 	require.Contains(t, rec.Body.String(), "positive integer")
 }
+
+// subscription_enabled is an opt-out switch: an explicit false is written as-is,
+// and a later payload that omits the field keeps the stored value.
+func TestUpdateSettingsSubscriptionEnabledIsWritableAndKeptWhenOmitted(t *testing.T) {
+	h, repo := newStepUpSwitchTestHandler(t, map[string]string{
+		service.SettingKeySubscriptionEnabled: "true",
+	})
+
+	rec := doUpdateSettings(t, h, map[string]any{"subscription_enabled": false}, nil)
+	require.Equal(t, http.StatusOK, rec.Code)
+	require.Equal(t, "false", repo.values[service.SettingKeySubscriptionEnabled])
+
+	rec = doUpdateSettings(t, h, map[string]any{"site_name": "Example Gateway"}, nil)
+	require.Equal(t, http.StatusOK, rec.Code)
+	require.Equal(t, "false", repo.values[service.SettingKeySubscriptionEnabled],
+		"a payload without subscription_enabled must not flip the stored value back to true")
+}

backend/internal/handler/admin/setting_handler_update.go

@@ -344,6 +344,9 @@ type UpdateSettingsRequest struct {
 	// Available Channels feature switch (user-facing)
 	AvailableChannelsEnabled *bool `json:"available_channels_enabled"`
 
+	// Subscription feature switch (user-facing subscription surface; see SettingKeySubscriptionEnabled)
+	SubscriptionEnabled *bool `json:"subscription_enabled"`
+
 	// Model Plaza feature switches + description
 	ModelPlazaEnabled     *bool   `json:"model_plaza_enabled"`
 	ModelPlazaRequireAuth *bool   `json:"model_plaza_require_auth"`
@@ -1937,6 +1940,12 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 			}
 			return previousSettings.AvailableChannelsEnabled
 		}(),
+		SubscriptionEnabled: func() bool {
+			if req.SubscriptionEnabled != nil {
+				return *req.SubscriptionEnabled
+			}
+			return previousSettings.SubscriptionEnabled
+		}(),
 		ModelPlazaEnabled: func() bool {
 			if req.ModelPlazaEnabled != nil {
 				return *req.ModelPlazaEnabled
@@ -2380,6 +2389,7 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		GrokDefaultBaseURLMode:         updatedSettings.GrokDefaultBaseURLMode,
 
 		AvailableChannelsEnabled: updatedSettings.AvailableChannelsEnabled,
+		SubscriptionEnabled:      updatedSettings.SubscriptionEnabled,
 
 		ModelPlazaEnabled:       updatedSettings.ModelPlazaEnabled,
 		ModelPlazaRequireAuth:   updatedSettings.ModelPlazaRequireAuth,

backend/internal/handler/dto/settings.go

@@ -317,6 +317,10 @@ type SystemSettings struct {
 	// Available Channels feature switch (user-facing aggregate view)
 	AvailableChannelsEnabled bool `json:"available_channels_enabled"`
 
+	// Subscription feature switch: gates the whole user-facing subscription surface
+	// (sidebar entries, purchase-page subscription tab, header badge, /subscriptions route).
+	SubscriptionEnabled bool `json:"subscription_enabled"`
+
 	// Model Plaza feature (public group/model pricing showcase)
 	ModelPlazaEnabled       bool   `json:"model_plaza_enabled"`
 	ModelPlazaRequireAuth   bool   `json:"model_plaza_require_auth"`
@@ -403,7 +407,11 @@ type PublicSettings struct {
 	GoogleOAuthEnabled                  bool                     `json:"google_oauth_enabled"`
 	BackendModeEnabled                  bool                     `json:"backend_mode_enabled"`
 	PaymentEnabled                      bool                     `json:"payment_enabled"`
-	Version                             string                   `json:"version"`
+	// PaymentBalanceDisabled mirrors the payment-config BALANCE_PAYMENT_DISABLED switch so the
+	// user shell can derive the site billing mode (recharge & subscription / recharge only /
+	// subscription only) before any authenticated checkout call.
+	PaymentBalanceDisabled bool   `json:"payment_balance_disabled"`
+	Version                string `json:"version"`
 	// 服务器全局时区（IANA 名称与当前 UTC 偏移，如 "Asia/Shanghai" / "+08:00"）。
 	// 高峰时段等按服务器本地时间判定的窗口，前端展示时据此标注，避免用户按浏览器本地时间误读。
 	ServerTimezone              string  `json:"server_timezone"`
@@ -422,6 +430,8 @@ type PublicSettings struct {
 
 	AvailableChannelsEnabled bool `json:"available_channels_enabled"`
 
+	SubscriptionEnabled bool `json:"subscription_enabled"`
+
 	ModelPlazaEnabled       bool `json:"model_plaza_enabled"`
 	ModelPlazaRequireAuth   bool `json:"model_plaza_require_auth"`
 	PluginManagementEnabled bool `json:"plugin_management_enabled"`

backend/internal/handler/setting_handler.go

@@ -95,6 +95,7 @@ func (h *SettingHandler) GetPublicSettings(c *gin.Context) {
 		GoogleOAuthEnabled:                  settings.GoogleOAuthEnabled,
 		BackendModeEnabled:                  settings.BackendModeEnabled,
 		PaymentEnabled:                      settings.PaymentEnabled,
+		PaymentBalanceDisabled:              settings.PaymentBalanceDisabled,
 		Version:                             h.version,
 		ServerTimezone:                      timezone.Name(),
 		ServerUTCOffset:                     timezone.UTCOffset(),
@@ -111,6 +112,7 @@ func (h *SettingHandler) GetPublicSettings(c *gin.Context) {
 		ChannelMonitorHideUserRanking:        settings.ChannelMonitorHideUserRanking,
 
 		AvailableChannelsEnabled: settings.AvailableChannelsEnabled,
+		SubscriptionEnabled:      settings.SubscriptionEnabled,
 
 		ModelPlazaEnabled:       settings.ModelPlazaEnabled,
 		ModelPlazaRequireAuth:   settings.ModelPlazaRequireAuth,