feat(030): Dataset Lifecycle Workspace — Stats Bar, split-view, inline-edit, bulk actions

Backend:
- Add MetricItem, StatsCounts, ColumnDescriptionUpdate, MetricDescriptionUpdate DTOs
- Add metric_count to DatasetItem and DatasetDetailResponse
- Add server-side filtering via ?filter=unmapped|mapped|linked|all
- Add PUT endpoints for column/metric description inline-edit
- Add HTML stripping validation (max 2000 chars, plain text)
- Add WebSocket dataset.updated event on task completion
- RBAC: plugin:migration:WRITE for mutations, READ for reads
- 14 pytest tests (stats, filter, metrics, inline-edit, validation, 502/503)

Frontend:
- Rewrite +page.svelte as split-view orchestrator (387 lines)
- StatsBar.svelte — 4 metric tiles with aria-pressed, server-side filter dispatch
- DatasetList.svelte — card grid with progress bars, checkboxes, search
- DatasetPreview.svelte — presentational detail panel (container fetches)
- ColumnsTable.svelte — inline-edit with type chips, bold/italic fallback
- MetricsTable.svelte — inline-edit with expression hints
- 52 vitest tests across 7 test files
- i18n: 11 EN + 11 RU keys (with_mapping/с_маппингом)

Spec:
- 15 issues resolved (semantic label, filtering, endpoints, conflict, metric_count, RBAC, ID types, ports, i18n, validation, a11y, propagation, resize, perf, ownership)

Status: 14/14 backend tests pass, 52/52 frontend tests pass, build succeeds
Risk: Low — T051 browser validation pending (non-blocking)

backend/src/app.py

@@ -393,6 +393,33 @@ async def websocket_endpoint(
                 extra={"task_id": task_id},
             )
 # #endregion websocket_endpoint
+# #region dataset_websocket_endpoint [C:4] [TYPE Function]
+# @BRIEF WebSocket endpoint for dataset.updated events — auto-refresh on task completion.
+# @RELATION CALLS -> [TaskManagerPackage]
+# @PRE env_id must reference a known environment.
+# @POST WebSocket streams dataset.updated events until disconnect.
+# @SIDE_EFFECT Subscribes to dataset event queue in task manager lifecycle.
+@app.websocket("/ws/datasets/{env_id}")
+async def dataset_websocket_endpoint(websocket: WebSocket, env_id: str):
+    seed_trace_id()
+    with belief_scope("dataset_websocket_endpoint", f"env_id={env_id}"):
+        await websocket.accept()
+        logger.reason("Accepted dataset event WebSocket", extra={"env_id": env_id})
+        task_manager = get_task_manager()
+        queue = await task_manager.subscribe_dataset_events(env_id)
+        try:
+            while True:
+                event = await queue.get()
+                await websocket.send_json(event)
+                logger.reflect("Forwarded dataset.updated event to client", extra={"env_id": env_id})
+        except WebSocketDisconnect:
+            logger.reason("WebSocket client disconnected from dataset events", extra={"env_id": env_id})
+        except Exception as exc:
+            logger.explore("WebSocket dataset streaming failed", extra={"env_id": env_id, "error": str(exc)})
+        finally:
+            task_manager.unsubscribe_dataset_events(env_id, queue)
+            logger.reflect("Released dataset event subscription", extra={"env_id": env_id})
+# #endregion dataset_websocket_endpoint
 # #region StaticFiles [C:1] [TYPE Mount] [SEMANTICS static, frontend, spa]
 # @BRIEF Mounts the frontend build directory to serve static assets.
 frontend_path = project_root / "frontend" / "build"