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/api/__init__.py

@@ -1,9 +1,17 @@
 """API blueprints."""
 from backend.app.api.auth import auth_bp
-from backend.app.api.c2 import c2_bp
+from backend.app.api.c2 import c2_bp, sims_c2_bp
 from backend.app.api.engagements import engagements_bp
 from backend.app.api.simulations import simulations_bp
 from backend.app.api.templates import templates_bp
 from backend.app.api.users import users_bp
 
-__all__ = ["auth_bp", "c2_bp", "users_bp", "engagements_bp", "simulations_bp", "templates_bp"]
+__all__ = [
+    "auth_bp",
+    "c2_bp",
+    "sims_c2_bp",
+    "users_bp",
+    "engagements_bp",
+    "simulations_bp",
+    "templates_bp",
+]