fix(routers+imports): fix missing DetectionRule import and correct -> list return type annotations that actually return paginated dicts

backend/app/routers/threat_actors.py

@@ -62,7 +62,7 @@ def list_threat_actors(
     db: Session = Depends(get_db),
     # Entry: current_user
     current_user: User = Depends(get_current_user),
-) -> list:
+) -> dict:
     """List threat actors with optional filters and pagination.
 
     **Requires** authentication (any role).
@@ -138,7 +138,7 @@ def get_threat_actor_gaps(
     db: Session = Depends(get_db),
     # Entry: current_user
     current_user: User = Depends(get_current_user),
-) -> list:
+) -> dict:
     """Identify techniques of this actor that are NOT fully validated.
 
     **Requires** authentication (any role).