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/routes/products.ts

@@ -39,8 +39,8 @@ router.post('/', async (req: AuthRequest, res: Response) => {
       return;
     }
 
-    // Scrape product info
-    const scrapedData = await scrapeProduct(url);
+    // Scrape product info (pass userId for AI fallback)
+    const scrapedData = await scrapeProduct(url, userId);
 
     // Allow adding out-of-stock products, but require a price for in-stock ones
     if (!scrapedData.price && scrapedData.stockStatus !== 'out_of_stock') {