feat(backend): c2 crypto + config CRUD + adapter scaffolding (sprint 8 M1)

- Add Fernet crypto service (MIMIC_ENCRYPTION_KEY env, C2Disabled on absent key)
- Add Alembic migration 0006: c2_config + c2_task tables with cascade FKs
- Add C2Config and C2Task SQLAlchemy models
- Add C2Adapter ABC with dataclasses (C2Health, C2Callback, C2TaskStatus, C2TaskPage)
- Add FakeAdapter (deterministic in-memory, MIMIC_C2_ADAPTER=fake)
- Add MythicAdapter scaffold: test_connection() live, M2+ raise NotImplementedError
- Add decode_response_text() helper for base64/binary Mythic responses
- Add GET/PUT/DELETE/POST-test /api/engagements/<id>/c2-config endpoints
- RBAC: admin+redteam OK, SOC 403; 503 guard when encryption key absent
- Token never returned in API responses; stored Fernet-encrypted only
- 42 new tests (300 total, 258 baseline preserved green)

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

backend/app/models/__init__.py

@@ -1,4 +1,6 @@
 """SQLAlchemy models."""
+from backend.app.models.c2_config import C2Config
+from backend.app.models.c2_task import C2Task, C2TaskSource
 from backend.app.models.engagement import Engagement, EngagementStatus
 from backend.app.models.simulation import Simulation, SimulationStatus
 from backend.app.models.simulation_template import SimulationTemplate
@@ -12,4 +14,7 @@ __all__ = [
     "Simulation",
     "SimulationStatus",
     "SimulationTemplate",
+    "C2Config",
+    "C2Task",
+    "C2TaskSource",
 ]