Add AI-powered price extraction fallback

- Add AI extraction service supporting Anthropic (Claude) and OpenAI
- Add AI settings UI in Settings page with provider selection
- Add database migration for AI settings columns
- Integrate AI fallback into scraper when standard methods fail
- Add API endpoints for AI settings and test extraction

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/services/scheduler.ts

@@ -23,7 +23,7 @@ async function checkPrices(): Promise<void> {
       try {
         console.log(`Checking price for product ${product.id}: ${product.url}`);
 
-        const scrapedData = await scrapeProduct(product.url);
+        const scrapedData = await scrapeProduct(product.url, product.user_id);
 
         // Check for back-in-stock notification
         const wasOutOfStock = product.stock_status === 'out_of_stock';