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/handler.go

@@ -0,0 +1,36 @@
+package tasks
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// Handler provides HTTP endpoints for task status
+type Handler struct {
+	manager *Manager
+}
+
+// NewHandler creates a new task handler
+func NewHandler(manager *Manager) *Handler {
+	return &Handler{
+		manager: manager,
+	}
+}
+
+// List returns all active and recent tasks
+func (h *Handler) List(c *gin.Context) {
+	tasks := h.manager.List()
+	c.JSON(http.StatusOK, gin.H{"tasks": tasks})
+}
+
+// Get returns a single task by ID
+func (h *Handler) Get(c *gin.Context) {
+	taskID := c.Param("id")
+	task, err := h.manager.Get(taskID)
+	if err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": "task not found"})
+		return
+	}
+	c.JSON(http.StatusOK, task)
+}