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/tasks/task.go

@@ -0,0 +1,34 @@
+package tasks
+
+import "time"
+
+// TaskStatus represents the current status of a task
+type TaskStatus string
+
+const (
+	TaskStatusRunning   TaskStatus = "running"
+	TaskStatusCompleted TaskStatus = "completed"
+	TaskStatusError     TaskStatus = "error"
+)
+
+// TaskType represents the type of background task
+type TaskType string
+
+const (
+	TaskTypeRecalculate     TaskType = "recalculate"
+	TaskTypeStockImport     TaskType = "stock_import"
+	TaskTypePricelistCreate TaskType = "pricelist_create"
+)
+
+// Task represents a background task with progress tracking
+type Task struct {
+	ID        string                 `json:"id"`
+	Type      TaskType               `json:"type"`
+	Status    TaskStatus             `json:"status"`
+	Progress  int                    `json:"progress"`   // 0-100
+	Message   string                 `json:"message"`
+	Result    map[string]interface{} `json:"result,omitempty"`
+	Error     string                 `json:"error,omitempty"`
+	CreatedAt time.Time              `json:"created_at"`
+	DoneAt    *time.Time             `json:"done_at,omitempty"`
+}