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>

a_long_dianjing/settings.py

@@ -56,6 +56,7 @@ MIDDLEWARE = [
     'django.contrib.auth.middleware.AuthenticationMiddleware',
     'django.contrib.messages.middleware.MessageMiddleware',
     'django.middleware.clickjacking.XFrameOptionsMiddleware',
+    'utils.slow_request_middleware.SlowRequestTraceMiddleware',
 ]
 
 # 日志配置 — 500 错误堆栈输出到 console，受 DEBUG_DETAIL_LOG 开关控制
@@ -126,7 +127,6 @@ DATABASES = {
         'PASSWORD': app_secrets.DATABASE_PASSWORD,
         'PORT': app_secrets.DATABASE_PORT,
         'CONN_MAX_AGE': 600,
-        'CONN_HEALTH_CHECKS': True,
         'OPTIONS': {
             'charset': 'utf8mb4',
             'init_command': "SET time_zone='+08:00'",