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/prices.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();
@@ -65,8 +65,11 @@ router.post('/:productId/refresh', async (req: AuthRequest, res: Response) => {
     // Scrape product data including price and stock status
     const scrapedData = await scrapeProduct(product.url);
 
-    // Update stock status
-    await productQueries.updateStockStatus(productId, scrapedData.stockStatus);
+    // Update stock status and record change if different
+    if (scrapedData.stockStatus !== product.stock_status) {
+      await productQueries.updateStockStatus(productId, scrapedData.stockStatus);
+      await stockStatusHistoryQueries.recordChange(productId, scrapedData.stockStatus);
+    }
 
     // Record new price if available
     let newPrice = null;
@@ -94,4 +97,38 @@ router.post('/:productId/refresh', async (req: AuthRequest, res: Response) => {
   }
 });
 
+// Get stock status history for a product
+router.get('/:productId/stock-history', async (req: AuthRequest, res: Response) => {
+  try {
+    const userId = req.userId!;
+    const productId = parseInt(req.params.productId, 10);
+
+    if (isNaN(productId)) {
+      res.status(400).json({ error: 'Invalid product ID' });
+      return;
+    }
+
+    // Verify product belongs to user
+    const product = await productQueries.findById(productId, userId);
+    if (!product) {
+      res.status(404).json({ error: 'Product not found' });
+      return;
+    }
+
+    // Get optional days filter from query (default 30 days)
+    const days = req.query.days ? parseInt(req.query.days as string, 10) : 30;
+
+    const stockHistory = await stockStatusHistoryQueries.getByProductId(productId, days);
+    const stats = await stockStatusHistoryQueries.getStats(productId, days);
+
+    res.json({
+      history: stockHistory,
+      stats,
+    });
+  } catch (error) {
+    console.error('Error fetching stock status history:', error);
+    res.status(500).json({ error: 'Failed to fetch stock status history' });
+  }
+});
+
 export default router;