fix: address deep code review issues for RPM limiting

- Move IncrementRPM after Forward success to prevent phantom RPM
  consumption during account switch retries
- Add base_rpm input sanitization (clamp to 0-10000) in Create/Update
- Add WindowCost scheduling checks to legacy path sticky sessions
  (4 check sites + 4 prefetch sites), fixing pre-existing gap
- Clean up rpm_strategy/rpm_sticky_buffer when disabling RPM in
  BulkEditModal (JSONB merge cannot delete keys, use empty values)
- Add json.Number test cases to TestGetBaseRPM/TestGetRPMStickyBuffer
- Document TOCTOU race as accepted soft-limit design trade-off

backend/internal/handler/admin/account_handler.go

@@ -483,6 +483,8 @@ func (h *AccountHandler) Create(c *gin.Context) {
 		response.BadRequest(c, "rate_multiplier must be >= 0")
 		return
 	}
+	// base_rpm 输入校验：负值归零，超过 10000 截断
+	sanitizeExtraBaseRPM(req.Extra)
 
 	// 确定是否跳过混合渠道检查
 	skipCheck := req.ConfirmMixedChannelRisk != nil && *req.ConfirmMixedChannelRisk
@@ -552,6 +554,8 @@ func (h *AccountHandler) Update(c *gin.Context) {
 		response.BadRequest(c, "rate_multiplier must be >= 0")
 		return
 	}
+	// base_rpm 输入校验：负值归零，超过 10000 截断
+	sanitizeExtraBaseRPM(req.Extra)
 
 	// 确定是否跳过混合渠道检查
 	skipCheck := req.ConfirmMixedChannelRisk != nil && *req.ConfirmMixedChannelRisk
@@ -1736,3 +1740,44 @@ func (h *AccountHandler) BatchRefreshTier(c *gin.Context) {
 func (h *AccountHandler) GetAntigravityDefaultModelMapping(c *gin.Context) {
 	response.Success(c, domain.DefaultAntigravityModelMapping)
 }
+
+// sanitizeExtraBaseRPM 对 extra map 中的 base_rpm 值进行范围校验和归一化。
+// 负值归零，超过 10000 截断为 10000。extra 为 nil 或不含 base_rpm 时无操作。
+func sanitizeExtraBaseRPM(extra map[string]any) {
+	if extra == nil {
+		return
+	}
+	raw, ok := extra["base_rpm"]
+	if !ok {
+		return
+	}
+	v := parseExtraIntForValidation(raw)
+	if v < 0 {
+		v = 0
+	} else if v > 10000 {
+		v = 10000
+	}
+	extra["base_rpm"] = v
+}
+
+// parseExtraIntForValidation 从 extra 字段的 any 值解析为 int，用于输入校验。
+// 支持 int, int64, float64, json.Number, string 类型。
+func parseExtraIntForValidation(value any) int {
+	switch v := value.(type) {
+	case int:
+		return v
+	case int64:
+		return int(v)
+	case float64:
+		return int(v)
+	case json.Number:
+		if i, err := v.Int64(); err == nil {
+			return int(i)
+		}
+	case string:
+		if i, err := strconv.Atoi(strings.TrimSpace(v)); err == nil {
+			return i
+		}
+	}
+	return 0
+}