revert: 移除 CONN_HEALTH_CHECKS（远程库每条请求多打一次 SQL）；加可选慢请求追踪

Co-authored-by: Cursor <cursoragent@cursor.com>

deploy/enable-slow-trace.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+# 开启慢请求追踪（只写日志，不改业务逻辑）。卡的时候跑这个，操作小程序，再 cat 日志。
+set -e
+SVC=/etc/systemd/system/gunicorn-dianjing.service
+mkdir -p /etc/systemd/system/gunicorn-dianjing.service.d
+cat > /etc/systemd/system/gunicorn-dianjing.service.d/slow-trace.conf <<'EOF'
+[Service]
+Environment=SLOW_REQUEST_TRACE=1
+Environment=SLOW_REQUEST_THRESHOLD_MS=200
+Environment=SLOW_REQUEST_LOG=/tmp/slow-request-trace.log
+EOF
+: > /tmp/slow-request-trace.log
+systemctl daemon-reload
+systemctl restart gunicorn-dianjing.service
+echo "已开启。现在去小程序复现卡顿，然后执行:"
+echo "  cat /tmp/slow-request-trace.log"
+echo "关闭: bash deploy/disable-slow-trace.sh"