Add stock status history tracking and timeline visualization

- Create stock_status_history table to track status changes over time
- Add stockStatusHistoryQueries with getByProductId, recordChange, getStats
- Update scheduler to record status changes
- Update product creation and manual refresh to record initial/changed status
- Add GET /products/:id/stock-history API endpoint
- Create StockTimeline component with:
  - Visual timeline bar showing in-stock (green) vs out-of-stock (red)
  - Availability percentage
  - Outage count and duration stats
- Integrate timeline into ProductDetail page

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

backend/src/routes/products.ts

@@ -1,6 +1,6 @@
 import { Router, Response } from 'express';
 import { AuthRequest, authMiddleware } from '../middleware/auth';
-import { productQueries, priceHistoryQueries } from '../models';
+import { productQueries, priceHistoryQueries, stockStatusHistoryQueries } from '../models';
 import { scrapeProduct } from '../services/scraper';
 
 const router = Router();
@@ -69,6 +69,11 @@ router.post('/', async (req: AuthRequest, res: Response) => {
       );
     }
 
+    // Record initial stock status
+    if (scrapedData.stockStatus !== 'unknown') {
+      await stockStatusHistoryQueries.recordChange(product.id, scrapedData.stockStatus);
+    }
+
     // Update last_checked timestamp and schedule next check
     await productQueries.updateLastChecked(product.id, product.refresh_interval);