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>

frontend/src/api/client.ts

@@ -61,12 +61,14 @@ export interface Product {
   last_checked: string | null;
   stock_status: StockStatus;
   price_drop_threshold: number | null;
+  target_price: number | null;
   notify_back_in_stock: boolean;
   created_at: string;
   current_price: number | null;
   currency: string | null;
   sparkline?: SparklinePoint[];
   price_change_7d?: number | null;
+  min_price?: number | null;
 }
 
 export interface ProductWithStats extends Product {
@@ -98,6 +100,7 @@ export const productsApi = {
     name?: string;
     refresh_interval?: number;
     price_drop_threshold?: number | null;
+    target_price?: number | null;
     notify_back_in_stock?: boolean;
   }) => api.put<Product>(`/products/${id}`, data),