Add anchor price support for variant products

When a user confirms a price from the modal, we now store it as an
"anchor price". On subsequent refreshes, if multiple price candidates
are found (common with variant products like different sizes/colors),
we select the candidate closest to the anchor price.

This fixes the issue where variant products would randomly switch to
a different variant's price on refresh.

Changes:
- Add anchor_price column to products table
- Save anchor price when user confirms a price selection
- Use anchor price to select correct variant on refresh
- 20% tolerance - if no candidate is within 20% of anchor, fall back to consensus

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

backend/src/routes/products.ts

@@ -57,6 +57,9 @@ router.post('/', async (req: AuthRequest, res: Response) => {
       // Store the preferred extraction method and the user-selected price
       await productQueries.updateExtractionMethod(product.id, selectedMethod);
 
+      // Store the anchor price - used on refresh to select the correct variant
+      await productQueries.updateAnchorPrice(product.id, selectedPrice);
+
       // Record the user-selected price
       await priceHistoryQueries.create(
         product.id,