feat: Multi-strategy price voting system with user selection

- Add multi-strategy voting: runs JSON-LD, site-specific, generic CSS,
  and AI extraction methods in parallel
- Implement consensus voting to select the correct price when methods agree
- Add AI arbitration when extraction methods disagree
- Add PriceSelectionModal for users to select correct price when ambiguous
- Store preferred extraction method per product for faster re-checks
- Add database columns for preferred_extraction_method and needs_price_review

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

backend/src/models/index.ts

@@ -551,6 +551,21 @@ export const productQueries = {
     );
     return result.rows;
   },
+
+  updateExtractionMethod: async (id: number, method: string): Promise<void> => {
+    await pool.query(
+      'UPDATE products SET preferred_extraction_method = $1, needs_price_review = false WHERE id = $2',
+      [method, id]
+    );
+  },
+
+  getPreferredExtractionMethod: async (id: number): Promise<string | null> => {
+    const result = await pool.query(
+      'SELECT preferred_extraction_method FROM products WHERE id = $1',
+      [id]
+    );
+    return result.rows[0]?.preferred_extraction_method || null;
+  },
 };
 
 // Price History types and queries