Scraper: process-wide lock, one scrape at a time

Concurrent manual scrapes were interleaving (two Chromium sessions on one
LXC) causing the page timeouts behind partial results. SCRAPE_LOCK guards
run_manual_scrape and process_queue; the trigger endpoint returns 409 when
busy, and the frontend keeps the job queued and retries after 30s.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/services/booking_scraper.py

@@ -12,6 +12,7 @@ Features:
 """
 
 import logging
+import threading
 import uuid
 from datetime import date, datetime, timedelta
 from decimal import Decimal
@@ -24,6 +25,11 @@ from .scraper_backends import ScraperBackend, PlaywrightLocalBackend, HotelData,
 
 logger = logging.getLogger(__name__)
 
+# One scrape at a time, process-wide: concurrent Chromium runs starve each
+# other of memory (page timeouts → partial results) and double the request
+# rate at Booking.com. Guards both manual scrapes and queue processing.
+SCRAPE_LOCK = threading.Lock()
+
 
 def get_scraper_backend(db: Session) -> ScraperBackend:
     """
@@ -445,6 +451,24 @@ async def run_manual_scrape(
     if to_date is None:
         to_date = from_date
 
+    if not SCRAPE_LOCK.acquire(blocking=False):
+        logger.warning(f"Manual scrape {from_date}..{to_date} refused — another scrape is running")
+        return {
+            'success': False,
+            'already_running': True,
+            'error': 'Another scrape is already running. Try again when it finishes.',
+        }
+    try:
+        return await _run_manual_scrape_locked(db, from_date, to_date)
+    finally:
+        SCRAPE_LOCK.release()
+
+
+async def _run_manual_scrape_locked(
+    db: Session,
+    from_date: date,
+    to_date: date
+) -> Dict[str, Any]:
     # Check if paused
     if await is_scraper_paused(db):
         return {
@@ -655,6 +679,20 @@ async def process_queue(db: Session) -> Dict[str, Any]:
     Returns:
         Dict with processing results
     """
+    if not SCRAPE_LOCK.acquire(blocking=False):
+        logger.warning("Queue processing skipped — another scrape is running")
+        return {
+            'success': False,
+            'already_running': True,
+            'error': 'Another scrape is already running.',
+        }
+    try:
+        return await _process_queue_locked(db)
+    finally:
+        SCRAPE_LOCK.release()
+
+
+async def _process_queue_locked(db: Session) -> Dict[str, Any]:
     # Check if paused
     if await is_scraper_paused(db):
         return {