fix: round-3 review fixes for RPM limiting

- Add sanitizeExtraBaseRPM to BulkUpdate handler (was missing)
- Add WindowCost scheduling checks to legacy non-sticky selection
  paths (4 sites), matching existing sticky + load-aware coverage
- Export ParseExtraInt from service package, remove duplicate
  parseExtraIntForValidation from admin handler

backend/internal/service/account.go

@@ -1274,6 +1274,12 @@ func parseExtraFloat64(value any) float64 {
 }
 
 // parseExtraInt 从 extra 字段解析 int 值
+// ParseExtraInt 从 extra 字段的 any 值解析为 int。
+// 支持 int, int64, float64, json.Number, string 类型，无法解析时返回 0。
+func ParseExtraInt(value any) int {
+	return parseExtraInt(value)
+}
+
 func parseExtraInt(value any) int {
 	switch v := value.(type) {
 	case int: