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/services/c2/__init__.py

@@ -0,0 +1,20 @@
+"""C2 adapter package. Import the factory from here."""
+from backend.app.services.c2.adapter import (
+    C2Adapter,
+    C2Callback,
+    C2Health,
+    C2TaskPage,
+    C2TaskStatus,
+    decode_response_text,
+)
+from backend.app.services.c2.factory import get_adapter
+
+__all__ = [
+    "C2Adapter",
+    "C2Callback",
+    "C2Health",
+    "C2TaskPage",
+    "C2TaskStatus",
+    "decode_response_text",
+    "get_adapter",
+]