feat: implement background task system with notifications

- Added background task manager with goroutine execution and panic recovery
- Replaced SSE streaming with background task execution for:
  * Price recalculation (RecalculateAll)
  * Stock import (ImportStockLog)
  * Pricelist creation (CreateWithProgress)
- Implemented unified polling for task status and DB connection in frontend
- Added task indicator in top bar showing running tasks count
- Added toast notifications for task completion/error
- Tasks automatically cleaned up after 10 minutes
- Tasks show progress (0-100%) with descriptive messages
- Updated handler constructors to receive task manager
- Added API endpoints for task status (/api/tasks, /api/tasks/:id)

Fixes issue with SSE disconnection on slow connections during long-running operations

internal/models/pricelist.go

@@ -64,7 +64,9 @@ type PricelistItem struct {
 	ManualPrice      *float64 `gorm:"type:decimal(12,2)" json:"manual_price,omitempty"`
 	MetaPrices       string   `gorm:"size:1000" json:"meta_prices,omitempty"`
 
-	// Virtual fields for display
+	// Virtual fields for display (not stored in qt_pricelist_items table)
+	// LotDescription and Category are populated via JOIN in SQL queries
+	// AvailableQty and Partnumbers are populated programmatically
 	LotDescription string   `gorm:"-" json:"lot_description,omitempty"`
 	Category       string   `gorm:"-" json:"category,omitempty"`
 	AvailableQty   *float64 `gorm:"-" json:"available_qty,omitempty"`