Fix scrape submission race condition allowing duplicate background tasks

Two rapid POST /scrape requests could both pass the lock check before
either background task acquired SCRAPE_LOCK, queuing two sequential
scrapes for the same date range. The second would hit Cloudflare after
the first already succeeded, causing repeated retries.

_SCRAPE_PENDING is set on submission and cleared when the task starts,
closing the gap between the 409 check and lock acquisition.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/api/competitors.py

@@ -10,6 +10,8 @@ from sqlalchemy import text
 from pydantic import BaseModel
 import logging
 
+import threading
+
 from database import get_db, SyncSessionLocal
 from auth import get_current_user
 from services import proxy as proxy_util
@@ -17,6 +19,11 @@ from services import proxy as proxy_util
 router = APIRouter()
 logger = logging.getLogger(__name__)
 
+# Set when a background task is submitted; cleared when the task starts executing.
+# Prevents duplicate scrapes caused by the race between the 409 check and the
+# background task acquiring SCRAPE_LOCK.
+_SCRAPE_PENDING = threading.Event()
+
 
 # ============================================
 # REQUEST/RESPONSE MODELS
@@ -358,6 +365,8 @@ def run_scrape_sync(from_date: date, to_date: date):
     import asyncio
     from services.booking_scraper import run_manual_scrape, cleanup_stale_batches
 
+    _SCRAPE_PENDING.clear()  # Task is now running — allow new submissions to queue
+
     db = SyncSessionLocal()
     try:
         # Clean up any stale batches before starting
@@ -407,17 +416,13 @@ async def trigger_manual_scrape(
     if not location_result.fetchone():
         raise HTTPException(status_code=400, detail="No scrape location configured. Set location first.")
 
-    # Only one scrape at a time — concurrent Chromium runs cause the page
-    # timeouts that produce partial results
-    #
-    # Best-effort early 409: the background task re-acquires the lock and will
-    # no-op (logging "another scrape is running") if it loses a millisecond-
-    # window race, so no double-run can slip through here.
+    # Only one scrape at a time — concurrent Chromium runs cause page timeouts.
+    # _SCRAPE_PENDING guards the window between submission and lock acquisition.
     from services.booking_scraper import get_lock_status
-    if get_lock_status()["locked"]:
+    if get_lock_status()["locked"] or _SCRAPE_PENDING.is_set():
         raise HTTPException(status_code=409, detail="A scrape is already running. Try again when it finishes.")
 
-    # Start background task
+    _SCRAPE_PENDING.set()
     background_tasks.add_task(run_scrape_sync, from_date, to_date)
 
     return {