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/factory.py

@@ -0,0 +1,19 @@
+"""Factory that resolves the C2Adapter implementation from MIMIC_C2_ADAPTER env."""
+from __future__ import annotations
+
+import os
+
+from backend.app.services.c2.adapter import C2Adapter
+
+
+def get_adapter(url: str, api_token: str, verify_tls: bool = True) -> C2Adapter:
+    """Return the correct C2Adapter based on MIMIC_C2_ADAPTER (default: mythic)."""
+    adapter_name = os.environ.get("MIMIC_C2_ADAPTER", "mythic").lower()
+
+    if adapter_name == "fake":
+        from backend.app.services.c2.fake import FakeAdapter
+        return FakeAdapter()
+
+    # Default: real Mythic adapter
+    from backend.app.services.c2.mythic import MythicAdapter
+    return MythicAdapter(url=url, api_token=api_token, verify_tls=verify_tls)