feat(affiliate): add feature toggle and per-user custom invite settings

- 在系统设置「功能开关」中新增邀请返利总开关，默认关闭；
  关闭态：菜单隐藏、注册忽略 aff、新充值不返利，但已有 quota 仍可转余额
- 支持管理员为指定用户设置专属邀请码（覆盖随机码，全局唯一）
- 支持管理员为指定用户设置专属返利比例（覆盖全局比例，可单条/批量调整）
- 在系统设置邀请返利卡片内嵌入专属用户管理表格（搜索/编辑/批量/删除），
  删除采用项目通用 ConfirmDialog，会同时清除专属比例并把邀请码重置为系统随机码
- /affiliate 用户页新增「我的返利比例」卡片与动态使用说明，让用户直观看到
  分享后能拿到多少（同源 resolveRebateRatePercent 计算，与实际充值一致）
- 新增数据库迁移 132 添加 aff_rebate_rate_percent 与 aff_code_custom 列
- 新增 admin 路由组 /api/v1/admin/affiliates/users/* 共 5 个端点
- AffiliateService 改为只依赖 *SettingService，去除冗余的 SettingRepository
- 邀请码格式校验放宽到 [A-Z0-9_-]{4,32}，兼容旧 12 位系统码与新自定义码
- 补充单元测试与集成测试覆盖新方法、冲突路径与边界值

backend/internal/service/setting_service.go

@@ -454,6 +454,7 @@ func (s *SettingService) GetPublicSettings(ctx context.Context) (*PublicSettings
 		SettingKeyChannelMonitorEnabled,
 		SettingKeyChannelMonitorDefaultIntervalSeconds,
 		SettingKeyAvailableChannelsEnabled,
+		SettingKeyAffiliateEnabled,
 	}
 
 	settings, err := s.settingRepo.GetMultiple(ctx, keys)
@@ -541,6 +542,8 @@ func (s *SettingService) GetPublicSettings(ctx context.Context) (*PublicSettings
 		ChannelMonitorDefaultIntervalSeconds: parseChannelMonitorInterval(settings[SettingKeyChannelMonitorDefaultIntervalSeconds]),
 
 		AvailableChannelsEnabled: settings[SettingKeyAvailableChannelsEnabled] == "true",
+
+		AffiliateEnabled: settings[SettingKeyAffiliateEnabled] == "true",
 	}, nil
 }
 
@@ -687,6 +690,7 @@ type PublicSettingsInjectionPayload struct {
 	ChannelMonitorEnabled                bool `json:"channel_monitor_enabled"`
 	ChannelMonitorDefaultIntervalSeconds int  `json:"channel_monitor_default_interval_seconds"`
 	AvailableChannelsEnabled             bool `json:"available_channels_enabled"`
+	AffiliateEnabled                     bool `json:"affiliate_enabled"`
 }
 
 // GetPublicSettingsForInjection returns public settings in a format suitable for HTML injection.
@@ -739,6 +743,7 @@ func (s *SettingService) GetPublicSettingsForInjection(ctx context.Context) (any
 		ChannelMonitorEnabled:                settings.ChannelMonitorEnabled,
 		ChannelMonitorDefaultIntervalSeconds: settings.ChannelMonitorDefaultIntervalSeconds,
 		AvailableChannelsEnabled:             settings.AvailableChannelsEnabled,
+		AffiliateEnabled:                     settings.AffiliateEnabled,
 	}, nil
 }
 
@@ -1205,6 +1210,9 @@ func (s *SettingService) buildSystemSettingsUpdates(ctx context.Context, setting
 	// Available channels feature switch
 	updates[SettingKeyAvailableChannelsEnabled] = strconv.FormatBool(settings.AvailableChannelsEnabled)
 
+	// Affiliate (邀请返利) feature switch
+	updates[SettingKeyAffiliateEnabled] = strconv.FormatBool(settings.AffiliateEnabled)
+
 	// Claude Code version check
 	updates[SettingKeyMinClaudeCodeVersion] = settings.MinClaudeCodeVersion
 	updates[SettingKeyMaxClaudeCodeVersion] = settings.MaxClaudeCodeVersion
@@ -1480,6 +1488,30 @@ func (s *SettingService) IsInvitationCodeEnabled(ctx context.Context) bool {
 	return value == "true"
 }
 
+// IsAffiliateEnabled 检查是否启用邀请返利功能（总开关）
+func (s *SettingService) IsAffiliateEnabled(ctx context.Context) bool {
+	value, err := s.settingRepo.GetValue(ctx, SettingKeyAffiliateEnabled)
+	if err != nil {
+		return false // 默认关闭
+	}
+	return value == "true"
+}
+
+// GetAffiliateRebateRatePercent 读取并 clamp 全局返利比例。
+// 解析失败、缺失或越界都回退到 AffiliateRebateRateDefault — 该比例从不抛错，
+// 调用方只关心一个可用的数值。
+func (s *SettingService) GetAffiliateRebateRatePercent(ctx context.Context) float64 {
+	raw, err := s.settingRepo.GetValue(ctx, SettingKeyAffiliateRebateRate)
+	if err != nil {
+		return AffiliateRebateRateDefault
+	}
+	rate, err := strconv.ParseFloat(strings.TrimSpace(raw), 64)
+	if err != nil || math.IsNaN(rate) || math.IsInf(rate, 0) {
+		return AffiliateRebateRateDefault
+	}
+	return clampAffiliateRebateRate(rate)
+}
+
 // IsPasswordResetEnabled 检查是否启用密码重置功能
 // 要求：必须同时开启邮件验证
 func (s *SettingService) IsPasswordResetEnabled(ctx context.Context) bool {
@@ -1771,6 +1803,9 @@ func (s *SettingService) InitializeDefaultSettings(ctx context.Context) error {
 		// Available channels feature (default disabled; opt-in)
 		SettingKeyAvailableChannelsEnabled: "false",
 
+		// Affiliate (邀请返利) feature (default disabled; opt-in)
+		SettingKeyAffiliateEnabled: "false",
+
 		// Claude Code version check (default: empty = disabled)
 		SettingKeyMinClaudeCodeVersion: "",
 		SettingKeyMaxClaudeCodeVersion: "",
@@ -2091,6 +2126,9 @@ func (s *SettingService) parseSettings(settings map[string]string) *SystemSettin
 	// Available channels feature (default: disabled; strict true)
 	result.AvailableChannelsEnabled = settings[SettingKeyAvailableChannelsEnabled] == "true"
 
+	// Affiliate (邀请返利) feature (default: disabled; strict true)
+	result.AffiliateEnabled = settings[SettingKeyAffiliateEnabled] == "true"
+
 	// Claude Code version check
 	result.MinClaudeCodeVersion = settings[SettingKeyMinClaudeCodeVersion]
 	result.MaxClaudeCodeVersion = settings[SettingKeyMaxClaudeCodeVersion]