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>

database/init.sql

@@ -17,10 +17,22 @@ CREATE TABLE IF NOT EXISTS products (
   image_url TEXT,
   refresh_interval INTEGER DEFAULT 3600,
   last_checked TIMESTAMP,
+  stock_status VARCHAR(20) DEFAULT 'unknown',
   created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
   UNIQUE(user_id, url)
 );
 
+-- Migration: Add stock_status column if it doesn't exist (for existing databases)
+DO $$
+BEGIN
+  IF NOT EXISTS (
+    SELECT 1 FROM information_schema.columns
+    WHERE table_name = 'products' AND column_name = 'stock_status'
+  ) THEN
+    ALTER TABLE products ADD COLUMN stock_status VARCHAR(20) DEFAULT 'unknown';
+  END IF;
+END $$;
+
 -- Price history table
 CREATE TABLE IF NOT EXISTS price_history (
   id SERIAL PRIMARY KEY,