Add out-of-stock detection and display

- Add stock_status column to products table (in_stock/out_of_stock/unknown)
- Detect out-of-stock status on Amazon by checking:
  - #availability text for "currently unavailable"
  - #outOfStock element presence
  - Missing "Add to Cart" button
- Add generic stock status detection for other sites
- Allow adding out-of-stock products (they just won't have a price)
- Update background scheduler to track stock status changes
- Display stock status badge in product list and detail pages
- Dim out-of-stock products in the dashboard
- Show "Currently Unavailable" badge instead of price when out of stock

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

frontend/src/api/client.ts

@@ -41,6 +41,8 @@ export const authApi = {
 };
 
 // Products API
+export type StockStatus = 'in_stock' | 'out_of_stock' | 'unknown';
+
 export interface SparklinePoint {
   price: number;
   recorded_at: string;
@@ -54,6 +56,7 @@ export interface Product {
   image_url: string | null;
   refresh_interval: number;
   last_checked: string | null;
+  stock_status: StockStatus;
   created_at: string;
   current_price: number | null;
   currency: string | null;