fix: address code review issues for RPM limiting feature

- Use TxPipeline (MULTI/EXEC) instead of Pipeline for atomic INCR+EXPIRE
- Filter negative values in GetBaseRPM(), update test expectation
- Add RPM batch query (GetRPMBatch) to account List API
- Add warn logs for RPM increment failures in gateway handler
- Reset enableRpmLimit on BulkEditAccountModal close
- Use union type 'tiered' | 'sticky_exempt' for rpmStrategy refs
- Add design decision comments for rdb.Time() RTT trade-off

backend/internal/service/account.go

@@ -1138,13 +1138,16 @@ func (a *Account) GetSessionIdleTimeoutMinutes() int {
 }
 
 // GetBaseRPM 获取基础 RPM 限制
-// 返回 0 表示未启用
+// 返回 0 表示未启用（负数视为无效配置，按 0 处理）
 func (a *Account) GetBaseRPM() int {
 	if a.Extra == nil {
 		return 0
 	}
 	if v, ok := a.Extra["base_rpm"]; ok {
-		return parseExtraInt(v)
+		val := parseExtraInt(v)
+		if val > 0 {
+			return val
+		}
 	}
 	return 0
 }