feat(backend): c2 callbacks + execute endpoints (sprint 8 M2)

- Add C2Error exception to adapter ABC
- Add promote_to_in_progress() helper to simulation_workflow (pending→in_progress)
- Flesh out MythicAdapter: list_callbacks() (GraphQL query) + create_task() (mutation)
- Expand FakeAdapter to 3 deterministic callbacks; switch task store to per-instance
- Add GET /api/engagements/<id>/c2/callbacks — lists active callbacks via adapter
- Add POST /api/simulations/<id>/c2/execute — issues tasks, stores C2Task rows,
  auto-transitions pending→in_progress, blocks on done (409)
- Both endpoints: SOC=403, 503 no-key, 502 adapter error, sanitized error messages
- Add requests-mock==1.12.1 to requirements.txt
- 42 new tests (342 total, 300 M1 baseline preserved green)

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

backend/app/services/simulation_workflow.py

@@ -98,6 +98,19 @@ def _maybe_activate_engagement(simulation: Simulation) -> None:
         db.session.add(engagement)
 
 
+def promote_to_in_progress(simulation: Simulation) -> None:
+    """Transition simulation pending → in_progress if it is currently pending.
+
+    Also advances the engagement planned → active via _maybe_activate_engagement.
+    No-op when the simulation is already in any other status.
+    Caller must commit.
+    """
+    if simulation.status == SimulationStatus.PENDING:
+        simulation.status = SimulationStatus.IN_PROGRESS
+        simulation.updated_at = datetime.now(UTC)
+        _maybe_activate_engagement(simulation)
+
+
 def apply_patch(
     simulation: Simulation, payload: dict[str, Any], user: User
 ) -> tuple[Any, int] | None: