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/crypto.py

@@ -0,0 +1,40 @@
+"""Fernet-based encryption service for sensitive fields.
+
+Key is read from the MIMIC_ENCRYPTION_KEY env var (Fernet base64-urlsafe 32-byte key).
+When the key is absent the service raises C2Disabled so callers can return 503.
+The key is never logged or returned in any response.
+"""
+from __future__ import annotations
+
+import os
+
+from cryptography.fernet import Fernet, InvalidToken
+
+
+class C2Disabled(Exception):
+    """Raised when MIMIC_ENCRYPTION_KEY is not set."""
+
+
+def _get_fernet() -> Fernet:
+    key = os.environ.get("MIMIC_ENCRYPTION_KEY")
+    if not key:
+        raise C2Disabled("C2 disabled: MIMIC_ENCRYPTION_KEY not set")
+    return Fernet(key.encode() if isinstance(key, str) else key)
+
+
+def encrypt(plaintext: str) -> str:
+    """Encrypt *plaintext* and return a Fernet token (str)."""
+    f = _get_fernet()
+    return f.encrypt(plaintext.encode()).decode()
+
+
+def decrypt(ciphertext: str) -> str:
+    """Decrypt a Fernet token and return the plaintext string."""
+    f = _get_fernet()
+    try:
+        return f.decrypt(ciphertext.encode()).decode()
+    except InvalidToken as exc:
+        raise ValueError("Invalid ciphertext") from exc
+
+
+__all__ = ["C2Disabled", "encrypt", "decrypt"]