Scraper: force-reset endpoint + watchdog to prevent stuck lock

Root cause: threading.Lock held indefinitely when Playwright browser
hangs inside run_in_executor (finally never fires from the async side).

Fixes:
- _acquire_scrape_lock/_release_scrape_lock track monotonic timestamp
- POST /competitors/scrape/reset force-releases the lock and marks any
  running batch as interrupted (queue rows stay intact for retry)
- GET /competitors/status now includes lock_held_seconds
- APScheduler watchdog job every 30 min auto-releases if held >3h
- Settings → Scraper Proxy tab shows live lock status (green/amber)
  with a Force Reset button requiring confirmation

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

backend/scheduler.py

@@ -68,6 +68,22 @@ async def run_scheduled_parity_check():
     await loop.run_in_executor(None, run_parity_check)
 
 
+async def run_scrape_watchdog():
+    """Auto-release the scrape lock if held for >3 hours (hung Playwright browser)."""
+    from services.booking_scraper import get_lock_status, force_reset_scraper
+    status = get_lock_status()
+    held = status.get("held_seconds")
+    if held and held > 3 * 3600:
+        logger.warning(f"Scrape watchdog: lock held for {held}s — force releasing")
+        db = SyncSessionLocal()
+        try:
+            force_reset_scraper(db)
+        except Exception as e:
+            logger.error(f"Scrape watchdog reset failed: {e}")
+        finally:
+            db.close()
+
+
 async def run_scheduled_booking_scrape_async():
     from jobs.scrape_booking_rates import run_scheduled_booking_scrape
     import asyncio
@@ -131,8 +147,17 @@ def start_scheduler():
         replace_existing=True,
     )
 
+    # Scrape lock watchdog — every 30 min; force-releases if held >3 hours
+    from apscheduler.triggers.interval import IntervalTrigger
+    scheduler.add_job(
+        run_scrape_watchdog,
+        IntervalTrigger(minutes=30),
+        id='scrape_watchdog',
+        replace_existing=True,
+    )
+
     scheduler.start()
-    logger.info(f"Scheduler started: booking scrape at {scrape_hour:02d}:{scrape_minute:02d}, rates fetch at {rates_hour:02d}:{rates_minute:02d}, direct scrape at 06:00, parity check at 06:45")
+    logger.info(f"Scheduler started: booking scrape at {scrape_hour:02d}:{scrape_minute:02d}, rates fetch at {rates_hour:02d}:{rates_minute:02d}, direct scrape at 06:00, parity check at 06:45, watchdog every 30m")
 
 
 def shutdown_scheduler():