feat(ops): 添加 count_tokens 错误标记数据库迁移

- 新增 is_count_tokens 布尔字段到 ops_error_logs 表
- 默认值为 false
- 支持后续动态过滤统计

backend/migrations/036_ops_error_logs_add_is_count_tokens.sql

+-- Migration: 添加 is_count_tokens 字段到 ops_error_logs 表
+-- Purpose: 标记 count_tokens 请求的错误，以便在统计和告警中根据配置动态过滤
+-- Author: System
+-- Date: 2026-01-12
+
+-- Add is_count_tokens column to ops_error_logs table
+ALTER TABLE ops_error_logs
+ADD COLUMN is_count_tokens BOOLEAN NOT NULL DEFAULT FALSE;
+
+-- Add comment
+COMMENT ON COLUMN ops_error_logs.is_count_tokens IS '是否为 count_tokens 请求的错误（用于统计过滤）';
+
+-- Create index for filtering (optional, improves query performance)
+CREATE INDEX IF NOT EXISTS idx_ops_error_logs_is_count_tokens
+ON ops_error_logs(is_count_tokens)
+WHERE is_count_tokens = TRUE;