Add target price alerts, historical low indicator, bulk actions, and dashboard summary

Features:
- Target price alerts: Set a specific price target and get notified when reached
- Historical low indicator: Badge showing when current price is at/near all-time low
- Bulk actions: Select multiple products to delete at once
- Dashboard summary: Shows total products, items at lowest price, at target, biggest drops

Backend changes:
- Add target_price column to products table
- Add target_price notification type with Telegram/Discord support
- Include min_price in product queries for historical low detection
- Update scheduler to check target price conditions

Frontend changes:
- Add target price input to ProductDetail notification settings
- Show target price badge on product cards
- Add "Lowest Price" and "Near Low" badges to product cards
- Add bulk selection mode with checkboxes
- Add dashboard summary cards at top of product list

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

database/init.sql

@@ -68,6 +68,7 @@ CREATE TABLE IF NOT EXISTS products (
   next_check_at TIMESTAMP,
   stock_status VARCHAR(20) DEFAULT 'unknown',
   price_drop_threshold DECIMAL(10,2),
+  target_price DECIMAL(10,2),
   notify_back_in_stock BOOLEAN DEFAULT false,
   created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
   UNIQUE(user_id, url)
@@ -107,6 +108,17 @@ BEGIN
   END IF;
 END $$;
 
+-- Migration: Add target_price column for price alerts
+DO $$
+BEGIN
+  IF NOT EXISTS (
+    SELECT 1 FROM information_schema.columns
+    WHERE table_name = 'products' AND column_name = 'target_price'
+  ) THEN
+    ALTER TABLE products ADD COLUMN target_price DECIMAL(10,2);
+  END IF;
+END $$;
+
 -- Price history table
 CREATE TABLE IF NOT EXISTS price_history (
   id SERIAL PRIMARY KEY,