fix(backend): sprint 4 post-review — relative paths + dead branch removal

- test_engagement_lifecycle.py, test_simulations_techniques.py: replace hardcoded
  absolute paths with Path(__file__).parent.parent / migrations/... (portable)
- simulation_workflow.py: remove dead branch in transition() — the IN_PROGRESS
  hook was unreachable since _ALLOWED_TRANSITIONS only targets review_required/done

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/simulation_workflow.py

@@ -225,10 +225,5 @@ def transition(
 
     simulation.status = SimulationStatus(to_status)
     simulation.updated_at = datetime.now(UTC)
-
-    # Hook: auto-activate engagement when simulation enters in_progress via manual transition.
-    if simulation.status == SimulationStatus.IN_PROGRESS:
-        _maybe_activate_engagement(simulation)
-
     db.session.commit()
     return None