feat(server): 设备上限登录闸(超限非硬拒登,返回 device_limit 信号)#16

启用设备数量限制的服务端部分。登录照常成功签发 token(非硬拒登),但若账户活跃
设备数超套餐上限,登录响应带 device_limit 信号,客户端据此弹「移除设备」页。

- devices/store.go:CountActiveDevices(last_seen 近 staleWindow)+ PruneStaleDevices
  (删超期僵尸行,免费版重装 churn 自愈)
- devices/service.go:staleWindow=30d;DeviceLimitStatus + CheckDeviceLimit
  (best-effort prune → ResolvePlan → 活跃 count > cap 即 Over,附活跃设备列表)
- auth:DeviceRegistrar 加 CheckDeviceLimit;recordLogin 回传 *DeviceLimit;
  LoginOutcome.DeviceLimit;Login 透传;handler tokenPairResponse.device_limit(omitempty)
- main.go:authDeviceRegistrar 适配 devices.CheckDeviceLimit → auth.DeviceLimit
- 测试:auth 登录透传超限信号(仍签发 token);devices within/over/prune-stale

连接侧 backstop(服务端硬拦)本轮从简未做,作为后续硬化(登录闸为客户端可信信号)。
DB 无 schema 变更。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/cmd/server/main.go

@@ -538,3 +538,22 @@ func (a authDeviceRegistrar) RegisterDevice(ctx context.Context, userID int64, m
 	}
 	return id, nil
 }
+
+// CheckDeviceLimit adapts devices.CheckDeviceLimit → auth.DeviceLimit for the login
+// over-limit gate. Returns nil (within cap) unless the account is over its plan cap.
+func (a authDeviceRegistrar) CheckDeviceLimit(ctx context.Context, userID int64) (*auth.DeviceLimit, error) {
+	st, apiErr := a.svc.CheckDeviceLimit(ctx, userID)
+	if apiErr != nil {
+		return nil, apiErr
+	}
+	if st == nil || !st.Over {
+		return nil, nil
+	}
+	briefs := make([]auth.DeviceBrief, 0, len(st.Devices))
+	for _, d := range st.Devices {
+		briefs = append(briefs, auth.DeviceBrief{
+			UUID: d.UUID, Name: d.Name, Platform: d.Platform, LastSeen: d.LastSeen,
+		})
+	}
+	return &auth.DeviceLimit{MaxDevices: st.MaxDevices, Devices: briefs}, nil
+}