merge: maestro/tsk_x7wrlA87orsY [devices + 订阅校验中间件] (tsk_VRzw-af__qWx)

手动合并 tsk_x7wrlA87orsY（设备管理 + 订阅校验中间件）到 main：

冲突解决：
- server/internal/apierr/apierr.go：保留 tsk_GXDoc3Cs07Rn 版本（New/StatusFor/
  Middleware/ErrConflict/改善文档），并入 tsk_x7wrlA87orsY 新增的 ErrAccountBanned
  及对应 StatusFor case（→ 403）。

新增文件（来自 tsk_x7wrlA87orsY）：
- server/internal/devices/doc.go       package 文档（替换占位 stub）
- server/internal/devices/context.go   CtxKeyUserID / Plan / WithPlan / PlanFromCtx
- server/internal/devices/handler.go   GET /v1/me/devices · DELETE /v1/me/devices/{id}
- server/internal/devices/middleware.go SubscriptionMiddleware · CheckDeviceQuota · RequirePaidTier
- server/internal/devices/service.go   RegisterIfAbsent / DeleteDevice / ResolvePlan + 纯函数 resolveEffectivePlan
- server/internal/devices/store.go     MySQL 数据访问层
- server/internal/devices/service_test.go          15 个单测（全通过）
- server/internal/devices/devices_integration_test.go  testcontainers 集成测试

OpenAPI 更新（来自 tsk_x7wrlA87orsY）：
- server/api/openapi.yaml：SubscriptionInfo.source 枚举补 free
- design/server/openapi.yaml：SubscriptionInfo.source 枚举补 admin, free

测试：go build ./... ✓；go test ./internal/apierr/... ✓（8 tests）；
      go test ./internal/devices/... ✓（15 tests）。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/internal/apierr/apierr.go

@@ -70,6 +70,11 @@ var (
 		MessageZH: "服务器内部错误，请稍后重试",
 		MessageEn: "Internal server error, please try again later",
 	}
+	ErrAccountBanned = &Error{
+		Code:      "ACCOUNT_BANNED",
+		MessageZH: "账户已被封禁，无法继续操作",
+		MessageEn: "This account has been banned",
+	}
 )
 
 // Activation-code errors.
@@ -137,6 +142,8 @@ func StatusFor(e *Error) int {
 		return http.StatusNotFound
 	case "CONFLICT":
 		return http.StatusConflict
+	case "ACCOUNT_BANNED":
+		return http.StatusForbidden
 	case "RATE_LIMITED", "ACCOUNT_LOCKED":
 		return http.StatusTooManyRequests
 	case "INTERNAL_ERROR":