feat: per-class autolabel params + frame-scoped class clear (REQ-180, REQ-181)

- REQ-181: class_params {name: {threshold?, iou_threshold?, min_box_frac?}}
  on /preview and /autolabel, per-class override table in both modals;
  empty overrides take the unchanged global path
- REQ-180: x button on each review sidebar class row clears that class on
  the current frame only via bulk-delete, no confirmation
- includes REQ-178 empty date-folder cycle fix (archive_index.py)

backend/api/batches.py

@@ -35,6 +35,7 @@ class AutolabelRequest(BaseModel):
     resume: bool = False
     append: bool = False
     custom_model_path: Optional[str] = None
+    class_params: Optional[dict[str, dict[str, float]]] = None
 
 class Exemplar(BaseModel):
     box: list[float]          # [cx, cy, w, h], normalized 0..1
@@ -53,6 +54,7 @@ class PreviewRequest(BaseModel):
     # Preview-only — `autolabel.start` deliberately has no equivalent.
     exemplars: Optional[list[Exemplar]] = None
     exemplar_class_name: Optional[str] = None
+    class_params: Optional[dict[str, dict[str, float]]] = None
 
 
 @router.post("/api/projects/{project_id}/batches")
@@ -116,7 +118,8 @@ def start_autolabel(batch_id: int, request: AutolabelRequest) -> dict:
                                engine=request.engine, engines=engine_list, class_ids=request.class_ids,
                                engine_classes=request.engine_classes,
                                target_class_names=request.target_class_names,
-                               custom_model_path=request.custom_model_path)
+                               custom_model_path=request.custom_model_path,
+                               class_params=request.class_params)
     except batch_store.BatchError as exc:
         raise HTTPException(400, str(exc))
 @router.post("/api/batches/inspect-model")
@@ -189,6 +192,7 @@ def preview_autolabel(batch_id: int, request: PreviewRequest) -> dict:
             custom_model_path=request.custom_model_path,
             exemplars=[e.model_dump() for e in request.exemplars or []],
             exemplar_class_name=request.exemplar_class_name,
+            class_params=request.class_params,
         )
         return {"shapes": shapes}
     except Exception as exc: