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/tests/test_c2_adapter_fake.py

@@ -0,0 +1,30 @@
+"""Tests for the FakeAdapter deterministic in-memory implementation."""
+from __future__ import annotations
+
+from backend.app.services.c2.adapter import C2Health
+from backend.app.services.c2.fake import FakeAdapter
+
+
+class TestFakeAdapterTestConnection:
+    def test_returns_ok_true(self):
+        adapter = FakeAdapter()
+        health = adapter.test_connection()
+        assert isinstance(health, C2Health)
+        assert health.ok is True
+        assert health.error is None
+
+    def test_list_callbacks_returns_list(self):
+        adapter = FakeAdapter()
+        callbacks = adapter.list_callbacks()
+        assert isinstance(callbacks, list)
+        assert len(callbacks) >= 1
+
+    def test_list_callbacks_fields(self):
+        adapter = FakeAdapter()
+        cb = adapter.list_callbacks()[0]
+        assert hasattr(cb, "display_id")
+        assert hasattr(cb, "active")
+        assert hasattr(cb, "host")
+        assert hasattr(cb, "user")
+        assert hasattr(cb, "domain")
+        assert hasattr(cb, "last_checkin")