v27.2: Fix /api/lab/status 422 crash — make batchId optional

GET /api/lab/status and /api/lab/result required batchId as a
mandatory query param, but the React SPA calls them with no param
on page load. FastAPI returned 422 Unprocessable Content, which
caused the BatchSimPage to crash and the UI to flash then vanish
in a retry loop.

Fix: batchId is now Optional[str] = None. When omitted, returns
the most recent active batch (or idle defaults for status).

lab_api.py

@@ -414,10 +414,24 @@ async def start_batch(req: StartRequest, background_tasks: BackgroundTasks):
 
 
 @app.get("/api/lab/status", response_model=StatusResponse)
-async def get_status(batchId: str):
+async def get_status(batchId: Optional[str] = None):
+    # If no batchId given, find the most recent active (or last) batch
+    state = None
+    if batchId:
         state = active_batches.get(batchId)
+    else:
+        # Return the most recently active batch, or any batch
+        for s in active_batches.values():
+            if s.running:
+                state = s
+                break
+        if state is None and active_batches:
+            state = list(active_batches.values())[-1]
     if not state:
-        raise HTTPException(404, f"Batch {batchId} not found")
+        return StatusResponse(
+            batchId="", running=False, completed=0, total=0,
+            threads=0, elapsedMs=0, error=None, simsPerSec=0.0,
+        )
     elapsed = int((datetime.now() - state.start_time).total_seconds() * 1000)
     return StatusResponse(
         batchId=state.batch_id,
@@ -432,10 +446,18 @@ async def get_status(batchId: str):
 
 
 @app.get("/api/lab/result")
-async def get_result(batchId: str):
+async def get_result(batchId: Optional[str] = None):
+    state = None
+    if batchId:
         state = active_batches.get(batchId)
+    else:
+        # Find most recent completed batch
+        for s in reversed(list(active_batches.values())):
+            if not s.running and s.result_path:
+                state = s
+                break
     if not state:
-        raise HTTPException(404, f"Batch {batchId} not found")
+        raise HTTPException(404, "No batch result available")
     if state.running:
         raise HTTPException(409, "Batch still running")
     if state.error: