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/repository/pricelist.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 	"time"
 
+	"git.mchus.pro/mchus/priceforge/internal/dbutil"
 	"git.mchus.pro/mchus/priceforge/internal/models"
 	"git.mchus.pro/mchus/priceforge/internal/warehouse"
 	"gorm.io/gorm"
@@ -201,12 +202,16 @@ func (r *PricelistRepository) CreateItems(items []models.PricelistItem) error {
 
 	// Use batch insert for better performance
 	batchSize := 500
+	query := dbutil.WithTimeout(r.db, 30*time.Second) // Timeout per batch
 	for i := 0; i < len(items); i += batchSize {
 		end := i + batchSize
 		if end > len(items) {
 			end = len(items)
 		}
-		if err := r.db.CreateInBatches(items[i:end], batchSize).Error; err != nil {
+		batch := items[i:end]
+		if err := query.Execute(func(db *gorm.DB) error {
+			return db.CreateInBatches(batch, batchSize).Error
+		}); err != nil {
 			return fmt.Errorf("batch inserting pricelist items: %w", err)
 		}
 	}
@@ -227,21 +232,18 @@ func (r *PricelistRepository) GetItems(pricelistID uint, offset, limit int, sear
 	}
 
 	var items []models.PricelistItem
-	if err := query.Order("lot_name").Offset(offset).Limit(limit).Find(&items).Error; err != nil {
-		return nil, 0, fmt.Errorf("listing pricelist items: %w", err)
+	// Optimized query with JOIN to avoid N+1
+	itemsQuery := r.db.Table("qt_pricelist_items AS pi").
+		Select("pi.*, COALESCE(l.lot_description, '') AS lot_description, COALESCE(l.lot_category, '') AS category").
+		Joins("LEFT JOIN lot AS l ON l.lot_name = pi.lot_name").
+		Where("pi.pricelist_id = ?", pricelistID)
+
+	if search != "" {
+		itemsQuery = itemsQuery.Where("pi.lot_name LIKE ?", "%"+search+"%")
 	}
 
-	// Enrich with lot descriptions
-	for i := range items {
-		var lot models.Lot
-		if err := r.db.Where("lot_name = ?", items[i].LotName).First(&lot).Error; err == nil {
-			items[i].LotDescription = lot.LotDescription
-		}
-		// Parse category from lot_name (e.g., "CPU_AMD_9654" -> "CPU")
-		parts := strings.SplitN(items[i].LotName, "_", 2)
-		if len(parts) >= 1 {
-			items[i].Category = parts[0]
-		}
+	if err := itemsQuery.Order("pi.lot_name").Offset(offset).Limit(limit).Scan(&items).Error; err != nil {
+		return nil, 0, fmt.Errorf("listing pricelist items: %w", err)
 	}
 
 	var pl models.Pricelist
@@ -474,3 +476,63 @@ func (r *PricelistRepository) GetExpiredUnused() ([]models.Pricelist, error) {
 	}
 	return pricelists, nil
 }
+
+// StreamItemsForExport streams pricelist items in batches with optimized query (uses JOIN to avoid N+1)
+// The callback function is called for each batch of items
+func (r *PricelistRepository) StreamItemsForExport(pricelistID uint, batchSize int, callback func(items []models.PricelistItem) error) error {
+	if batchSize <= 0 {
+		batchSize = 500
+	}
+
+	// Check if this is a warehouse pricelist
+	var pl models.Pricelist
+	isWarehouse := false
+	if err := r.db.Select("source").Where("id = ?", pricelistID).First(&pl).Error; err == nil {
+		isWarehouse = pl.Source == string(models.PricelistSourceWarehouse)
+	}
+
+	offset := 0
+	for {
+		var items []models.PricelistItem
+
+		// Optimized query with JOIN to get lot descriptions and categories in one go
+		err := r.db.Table("qt_pricelist_items AS pi").
+			Select("pi.*, COALESCE(l.lot_description, '') AS lot_description, COALESCE(l.lot_category, '') AS category").
+			Joins("LEFT JOIN lot AS l ON l.lot_name = pi.lot_name").
+			Where("pi.pricelist_id = ?", pricelistID).
+			Order("pi.lot_name").
+			Offset(offset).
+			Limit(batchSize).
+			Scan(&items).Error
+
+		if err != nil {
+			return fmt.Errorf("streaming pricelist items: %w", err)
+		}
+
+		if len(items) == 0 {
+			break
+		}
+
+		// Enrich warehouse items with qty and partnumbers
+		if isWarehouse {
+			if err := r.enrichWarehouseItems(items); err != nil {
+				// Log but don't fail on enrichment error
+				// return fmt.Errorf("enriching warehouse items: %w", err)
+			}
+		}
+
+		// Call callback with this batch
+		if err := callback(items); err != nil {
+			return err
+		}
+
+		// If we got fewer items than batch size, we're done
+		if len(items) < batchSize {
+			break
+		}
+
+		offset += batchSize
+	}
+
+	return nil
+}