Add staggered checking with jitter to prevent rate limiting

- Add next_check_at column to track when each product should be checked
- New products get random initial delay (0 to refresh_interval) to spread them out
- Each check adds ±5 minute jitter so products naturally drift apart over time
- Randomize delay between requests (2-5 seconds instead of fixed 2s)

This prevents all products from being checked at the same time,
reducing the risk of being rate-limited or blocked by retailers.

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

database/init.sql

@@ -33,6 +33,7 @@ CREATE TABLE IF NOT EXISTS products (
   image_url TEXT,
   refresh_interval INTEGER DEFAULT 3600,
   last_checked TIMESTAMP,
+  next_check_at TIMESTAMP,
   stock_status VARCHAR(20) DEFAULT 'unknown',
   price_drop_threshold DECIMAL(10,2),
   notify_back_in_stock BOOLEAN DEFAULT false,
@@ -63,6 +64,17 @@ BEGIN
   END IF;
 END $$;
 
+-- Migration: Add next_check_at column for staggered checking
+DO $$
+BEGIN
+  IF NOT EXISTS (
+    SELECT 1 FROM information_schema.columns
+    WHERE table_name = 'products' AND column_name = 'next_check_at'
+  ) THEN
+    ALTER TABLE products ADD COLUMN next_check_at TIMESTAMP;
+  END IF;
+END $$;
+
 -- Price history table
 CREATE TABLE IF NOT EXISTS price_history (
   id SERIAL PRIMARY KEY,