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/__init__.py

@@ -6,7 +6,15 @@ from pathlib import Path
 
 from flask import Flask, jsonify, send_from_directory
 
-from backend.app.api import auth_bp, c2_bp, engagements_bp, simulations_bp, templates_bp, users_bp
+from backend.app.api import (
+    auth_bp,
+    c2_bp,
+    engagements_bp,
+    sims_c2_bp,
+    simulations_bp,
+    templates_bp,
+    users_bp,
+)
 from backend.app.cli import register_cli
 from backend.app.config import Config, TestConfig
 from backend.app.errors import register_error_handlers
@@ -39,6 +47,7 @@ def create_app(config_object: object | None = None) -> Flask:
     app.register_blueprint(simulations_bp)
     app.register_blueprint(templates_bp)
     app.register_blueprint(c2_bp)
+    app.register_blueprint(sims_c2_bp)
 
     from backend.app.services import mitre as mitre_svc
     mitre_svc.load_bundle()