- 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
