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/internal/devices/devices_integration_test.go

@@ -265,6 +265,55 @@ func TestDeviceLimitEnforced(t *testing.T) {
 	}
 }
 
+// TestCheckDeviceLimit covers the login-gate helper: within-cap, over-cap (active
+// devices exceed the plan cap), and stale-prune (churned rows dropped + excluded).
+func TestCheckDeviceLimit(t *testing.T) {
+	db := setupMySQL(t)
+	svc := devices.NewService(devices.NewStore(db), nil)
+	ctx := context.Background()
+	userID := createUser(t, db, "cap@example.com", "active") // free plan, cap 1
+
+	// 1 device → within cap.
+	d1 := newUUID(t, db)
+	if _, _, e := svc.RegisterIfAbsent(ctx, devices.RegisterInput{UserID: userID, DeviceUUID: d1, Name: "P1", Platform: "android"}); e != nil {
+		t.Fatalf("register d1: %v", e)
+	}
+	st, apiErr := svc.CheckDeviceLimit(ctx, userID)
+	if apiErr != nil {
+		t.Fatalf("CheckDeviceLimit: %v", apiErr)
+	}
+	if st.Over || st.MaxDevices != 1 {
+		t.Fatalf("1 device should be within cap 1, got %+v", st)
+	}
+
+	// 2nd device (MaxDevices:0 bypasses the register-time cap) → over cap.
+	d2 := newUUID(t, db)
+	if _, _, e := svc.RegisterIfAbsent(ctx, devices.RegisterInput{UserID: userID, DeviceUUID: d2, Name: "P2", Platform: "ios"}); e != nil {
+		t.Fatalf("register d2: %v", e)
+	}
+	st, _ = svc.CheckDeviceLimit(ctx, userID)
+	if !st.Over || len(st.Devices) != 2 {
+		t.Fatalf("2 devices should exceed cap 1, got %+v", st)
+	}
+
+	// Age d1 past staleWindow → pruned + excluded → back within cap.
+	if _, err := db.Exec(`UPDATE devices SET last_seen=? WHERE uuid=?`,
+		time.Now().UTC().Add(-40*24*time.Hour), d1); err != nil {
+		t.Fatalf("age d1: %v", err)
+	}
+	st, _ = svc.CheckDeviceLimit(ctx, userID)
+	if st.Over {
+		t.Fatalf("stale device should be pruned/excluded → within cap, got %+v", st)
+	}
+	var n int
+	if err := db.QueryRow(`SELECT COUNT(1) FROM devices WHERE uuid=?`, d1).Scan(&n); err != nil {
+		t.Fatalf("count d1: %v", err)
+	}
+	if n != 0 {
+		t.Errorf("stale device should be pruned, still present")
+	}
+}
+
 // TestDeleteOthersDevice verifies ownership enforcement and not-found handling.
 func TestDeleteOthersDevice(t *testing.T) {
 	db := setupMySQL(t)