fix: ensure scheduler starts only once during Flask requests

web/app.py

@@ -1,4 +1,5 @@
 import os
+import threading
 from flask import Flask, request, jsonify, render_template, redirect, url_for, session, flash, Response
 from flask_wtf import CSRFProtect
 from typing import Dict, List
@@ -60,6 +61,10 @@ app.static_folder = "static"
 csrf = CSRFProtect(app)
 
 
+_scheduler_started = False
+_scheduler_lock = threading.Lock()
+
+
 def _scheduler_enabled() -> bool:
     flag = (os.environ.get("SCRAPE_SCHEDULER_ENABLED") or "").strip().lower()
     if flag not in {"1", "true", "yes", "on"}:
@@ -74,10 +79,22 @@ def _scheduler_enabled() -> bool:
     return True
 
 
-@app.before_first_request
-def _start_scheduler_if_enabled():
-    if _scheduler_enabled():
+def _maybe_start_scheduler() -> None:
+    global _scheduler_started
+    if _scheduler_started:
+        return
+    if not _scheduler_enabled():
+        return
+    with _scheduler_lock:
+        if _scheduler_started:
+            return
         start_scheduler_in_background()
+        _scheduler_started = True
+
+
+@app.before_request
+def _start_scheduler_if_enabled():
+    _maybe_start_scheduler()
 
 
 def require_admin():