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>

backend/src/models/index.ts

@@ -200,6 +200,7 @@ export interface Product {
   next_check_at: Date | null;
   stock_status: StockStatus;
   price_drop_threshold: number | null;
+  target_price: number | null;
   notify_back_in_stock: boolean;
   created_at: Date;
 }
@@ -222,6 +223,7 @@ export interface SparklinePoint {
 export interface ProductWithSparkline extends ProductWithLatestPrice {
   sparkline: SparklinePoint[];
   price_change_7d: number | null;
+  min_price: number | null;
 }
 
 export const productQueries = {
@@ -272,6 +274,15 @@ export const productQueries = {
       [productIds]
     );
 
+    // Get min prices for all products (all-time low)
+    const minPriceResult = await pool.query(
+      `SELECT product_id, MIN(price) as min_price
+       FROM price_history
+       WHERE product_id = ANY($1)
+       GROUP BY product_id`,
+      [productIds]
+    );
+
     // Group sparkline data by product
     const sparklineMap = new Map<number, SparklinePoint[]>();
     for (const row of sparklineResult.rows) {
@@ -280,6 +291,12 @@ export const productQueries = {
       sparklineMap.set(row.product_id, points);
     }
 
+    // Map min prices by product
+    const minPriceMap = new Map<number, number>();
+    for (const row of minPriceResult.rows) {
+      minPriceMap.set(row.product_id, parseFloat(row.min_price));
+    }
+
     // Combine products with sparkline data
     return products.map((product: ProductWithLatestPrice) => {
       const sparkline = sparklineMap.get(product.id) || [];
@@ -297,6 +314,7 @@ export const productQueries = {
         ...product,
         sparkline,
         price_change_7d: priceChange7d,
+        min_price: minPriceMap.get(product.id) || null,
       };
     });
   },
@@ -344,6 +362,7 @@ export const productQueries = {
       name?: string;
       refresh_interval?: number;
       price_drop_threshold?: number | null;
+      target_price?: number | null;
       notify_back_in_stock?: boolean;
     }
   ): Promise<Product | null> => {
@@ -363,6 +382,10 @@ export const productQueries = {
       fields.push(`price_drop_threshold = $${paramIndex++}`);
       values.push(updates.price_drop_threshold);
     }
+    if (updates.target_price !== undefined) {
+      fields.push(`target_price = $${paramIndex++}`);
+      values.push(updates.target_price);
+    }
     if (updates.notify_back_in_stock !== undefined) {
       fields.push(`notify_back_in_stock = $${paramIndex++}`);
       values.push(updates.notify_back_in_stock);