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/usage.go

@@ -7,6 +7,7 @@ import (
 	"github.com/gin-gonic/gin"
 	"github.com/openteam/server/internal/pkg/resp"
 	"github.com/openteam/server/internal/store"
+	"github.com/shopspring/decimal"
 )
 
 // UsageSummary GET /api/v1/usage/summary — 今日/本月汇总。
@@ -18,7 +19,7 @@ func (h *Handler) UsageSummary(c *gin.Context) {
 
 	var todayReq, monthReq int64
 	var todayTok, monthTok int64
-	var todayCost, monthCost float64
+	var todayCost, monthCost decimal.Decimal
 	h.a.DB.Model(&store.UsageDaily{}).Where("user_id = ? AND date = ?", u.ID, today).
 		Select("COALESCE(SUM(requests),0), COALESCE(SUM(input_tokens+output_tokens+cache_read_tokens),0), COALESCE(SUM(cost),0)").
 		Row().Scan(&todayReq, &todayTok, &todayCost)
@@ -47,7 +48,7 @@ func (h *Handler) UsageStats(c *gin.Context) {
 			ModelID  uint64
 			Requests int64
 			Tokens   int64
-			Cost     float64
+			Cost     decimal.Decimal
 		}
 		q.Select("model_id, SUM(requests) requests, SUM(input_tokens+output_tokens+cache_read_tokens) tokens, SUM(cost) cost").
 			Group("model_id").Scan(&rows)
@@ -64,7 +65,7 @@ func (h *Handler) UsageStats(c *gin.Context) {
 			Date     string
 			Requests int64
 			Tokens   int64
-			Cost     float64
+			Cost     decimal.Decimal
 		}
 		q.Select("date, SUM(requests) requests, SUM(input_tokens+output_tokens+cache_read_tokens) tokens, SUM(cost) cost").
 			Group("date").Order("date").Scan(&rows)