feat(ops): 添加上游错误事件数据库表

- 新建ops_upstream_error_events表存储上游服务错误详情
- 记录上游错误的请求ID、平台、模型、状态码等信息
- 支持索引优化查询性能（request_id, platform, status_code, created_at）

backend/migrations/034_ops_upstream_error_events.sql

+-- Add upstream error events list (JSONB) to ops_error_logs for per-request correlation.
+--
+-- This is intentionally idempotent.
+
+ALTER TABLE ops_error_logs
+    ADD COLUMN IF NOT EXISTS upstream_errors JSONB;
+
+COMMENT ON COLUMN ops_error_logs.upstream_errors IS
+    'Sanitized upstream error events list (JSON array), correlated per gateway request (request_id/client_request_id); used for per-request upstream debugging.';