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>

utils/slow_request_middleware.py

@@ -0,0 +1,47 @@
+"""
+可选：慢请求追踪（默认关闭，不影响现网）。
+启用: 在 gunicorn systemd 的 Environment 加 SLOW_REQUEST_TRACE=1 后 restart。
+日志: /tmp/slow-request-trace.log
+"""
+import logging
+import os
+import time
+
+logger = logging.getLogger('slow_request')
+
+_ENABLED = os.environ.get('SLOW_REQUEST_TRACE', '').strip() in ('1', 'true', 'yes')
+_THRESHOLD_MS = float(os.environ.get('SLOW_REQUEST_THRESHOLD_MS', '300'))
+_LOG_PATH = os.environ.get('SLOW_REQUEST_LOG', '/tmp/slow-request-trace.log')
+
+
+class SlowRequestTraceMiddleware:
+    def __init__(self, get_response):
+        self.get_response = get_response
+
+    def __call__(self, request):
+        if not _ENABLED:
+            return self.get_response(request)
+
+        t0 = time.perf_counter()
+        from django.db import connection, reset_queries
+        reset_queries()
+        response = self.get_response(request)
+        elapsed_ms = (time.perf_counter() - t0) * 1000
+
+        if elapsed_ms >= _THRESHOLD_MS:
+            n_sql = len(connection.queries)
+            sql_ms = sum(float(q.get('time', 0)) for q in connection.queries) * 1000
+            line = (
+                f"{elapsed_ms:.0f}ms total | sql={n_sql}条/{sql_ms:.0f}ms | "
+                f"{request.method} {request.path} | status={response.status_code}\n"
+            )
+            try:
+                with open(_LOG_PATH, 'a', encoding='utf-8') as f:
+                    f.write(line)
+                    if n_sql <= 30:
+                        for q in sorted(connection.queries, key=lambda x: -float(x.get('time', 0)))[:5]:
+                            sql = q['sql'][:200].replace('\n', ' ')
+                            f.write(f"  {float(q['time'])*1000:.0f}ms {sql}\n")
+            except OSError:
+                logger.warning('slow request trace write failed')
+        return response