Label Issue to refine the code by optimising it

[kavyashri-as]

Overview\n\nThis issue tracks code refinement and optimisation work for the Label feature.\n\n## Areas to Optimise\n\n- Database queries: Batch-fetch labels for all tasks in a single query instead of one query per task in enrichTasksWithLabels, reducing N+1 query overhead.\n- Frontend rendering: Avoid full task list re-renders on label assign/remove; update only the affected task DOM node.\n- Filter logic: Cache filtered task results client-side to avoid redundant API calls when switching between the same label filters.\n- Label validation: Consolidate duplicated validation logic between the test routes and the main index.js routes.\n\n## Acceptance Criteria\n\n- [ ] enrichTasksWithLabels uses a single batched SQL query\n- [ ] Assigning/removing a label updates only the relevant task in the DOM\n- [ ] No redundant network requests when re-selecting an active filter\n- [ ] Shared validation helpers are reused across route files and tests