feat(stats): #10 第②层 —— 统计页按设备过滤(per-device 本地日曲线)

镜像账户级时区曲线到「按设备」:
- migration 000018 `usage_device_hourly`(每设备 UTC 小时桶,稀疏)。
- ReportUsage deviceID>0 时加 AccumulateDeviceHourly;NodeStore 接口+impl+mock。
- usage.Store.DeviceHourlyRange(JOIN devices 校验归属+解析 uuid→id);UsageCurve
  增 deviceUUID 形参:空=账户级,非空=该设备本地日曲线(分桶逻辑复用)。
- /v1/usage?device=<uuid>;客户端 account_api.usage(device)、usageProvider key 改
  记录 (days,device)、stats_page 接 statsDeviceProvider → 选设备即重取该设备曲线。
测试:per-device 曲线隔离+归属校验(A的设备不进B)、UsageCurve(deviceUUID)、
migration v18、客户端 widget(选设备→/v1/usage 带 device=)。全量 go/flutter 绿。

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

server/internal/store/sqlite_migrate_test.go

@@ -29,8 +29,8 @@ func TestSQLiteMigrateUpDown(t *testing.T) {
 	if dirty {
 		t.Fatalf("schema dirty after MigrateUp")
 	}
-	if v != 17 {
-		t.Errorf("version = %d, want 17", v)
+	if v != 18 {
+		t.Errorf("version = %d, want 18", v)
 	}
 
 	// 2. Core tables exist.
@@ -38,7 +38,7 @@ func TestSQLiteMigrateUpDown(t *testing.T) {
 		"users", "devices", "plans", "subscriptions", "code_batches", "codes",
 		"usage_daily", "audit_log", "providers", "nodes", "node_events",
 		"directory_version", "provision_idempotency", "replacements", "admins",
-		"connect_credentials", "usage_device_daily", "sessions", "usage_hourly",
+		"connect_credentials", "usage_device_daily", "sessions", "usage_hourly", "usage_device_hourly",
 	} {
 		var name string
 		err := db.QueryRow(