feat: skip track forward on result overlap, not class presence (REQ-189)

Frame holding the tracked class is now annotated like any other; the run's
result is dropped only when it intersects a same-class shape already there.

backend/api/review.py

@@ -38,6 +38,8 @@ class AssistRequest(BaseModel):
     box: List[float]
     class_id: int = 0
     threshold: float = 0.5
+    # Track Forward only: drop the result when it lands on an existing shape.
+    dedupe: bool = False
 
 
 class PoolExemplar(BaseModel):
@@ -116,7 +118,7 @@ def bulk_reclass(request: BulkReclassRequest) -> dict:
 def assist(frame_id: int, request: AssistRequest) -> dict:
     try:
         return review_store.assist(frame_id, request.box, request.class_id,
-                                   request.threshold)
+                                   request.threshold, dedupe=request.dedupe)
     except review_store.ReviewError as exc:
         raise HTTPException(400, str(exc))