feat: 金额 decimal 精确计费、failover 按失败类型封顶、渠道小时桶预聚合、分层守护测试

- 计费：19 个金额字段 float64→decimal.Decimal，DB 列保持 numeric(20,8) 零迁移；
  API 金额仍为 JSON number（MarshalJSONWithoutQuotes），前端零改动
- failover：保留时间预算，新增 OT_PROXY_FAILOVER_CAPS_NET/SERVER/RATE/QUOTA
  按失败类型封顶换渠道次数（429/配额类瞬时失败时间预算形同虚设）
- 预聚合：新增 usage_channel_hourlies（schema v10），记账事务内同步 upsert，
  长窗口渠道统计走桶不再扫明细，spent_last_30d 改扫 usage_daily，v10 一次性回填幂等
- 守护：internal/arch_test.go 用 go/parser 强制分层依赖（fail-closed）

server/internal/api/keys_test.go

@@ -13,6 +13,7 @@ import (
 	"github.com/openteam/server/internal/api/middleware"
 	"github.com/openteam/server/internal/app"
 	"github.com/openteam/server/internal/store"
+	"github.com/shopspring/decimal"
 	"gorm.io/gorm"
 )
 
@@ -37,7 +38,7 @@ func newKeysTestEnv(t *testing.T) *keysTestEnv {
 		sqlDB.Close()
 	})
 	u := store.User{Username: "u1", Email: "u1@x.com", PasswordHash: "x",
-		Role: store.RoleUser, Balance: 100, Status: store.UserStatusActive}
+		Role: store.RoleUser, Balance: decimal.NewFromInt(100), Status: store.UserStatusActive}
 	if err := db.Create(&u).Error; err != nil {
 		t.Fatal(err)
 	}
@@ -126,7 +127,7 @@ func TestKeysLifecycle(t *testing.T) {
 func TestKeysUserIsolation(t *testing.T) {
 	e := newKeysTestEnv(t)
 	other := store.User{Username: "u2", Email: "u2@x.com", PasswordHash: "x",
-		Role: store.RoleUser, Balance: 0, Status: store.UserStatusActive}
+		Role: store.RoleUser, Balance: decimal.Zero, Status: store.UserStatusActive}
 	e.db.Create(&other)
 	var k store.APIKey
 	k = store.APIKey{UserID: other.ID, Name: "theirs", KeyHash: "h2", KeyPrefix: "pre",