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/services/scheduler.ts

@@ -26,11 +26,15 @@ async function checkPrices(): Promise<void> {
         // Get preferred extraction method for this product (if user previously selected one)
         const preferredMethod = await productQueries.getPreferredExtractionMethod(product.id);
 
-        // Use voting scraper with preferred method if available
+        // Get anchor price for variant products (the price the user confirmed)
+        const anchorPrice = await productQueries.getAnchorPrice(product.id);
+
+        // Use voting scraper with preferred method and anchor price if available
         const scrapedData = await scrapeProductWithVoting(
           product.url,
           product.user_id,
-          preferredMethod as ExtractionMethod | undefined
+          preferredMethod as ExtractionMethod | undefined,
+          anchorPrice || undefined
         );
 
         // Check for back-in-stock notification