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/adapter.py

@@ -0,0 +1,97 @@
+"""Abstract C2 adapter interface and shared dataclasses."""
+from __future__ import annotations
+
+import base64
+import binascii
+from abc import ABC, abstractmethod
+from dataclasses import dataclass
+
+
+@dataclass
+class C2Health:
+    ok: bool
+    error: str | None = None
+
+
+@dataclass
+class C2Callback:
+    display_id: int
+    active: bool
+    host: str
+    user: str
+    domain: str
+    last_checkin: str  # ISO-8601 string
+
+
+@dataclass
+class C2TaskStatus:
+    display_id: int
+    status: str
+    completed: bool
+
+
+@dataclass
+class C2TaskPage:
+    items: list[dict]  # raw task dicts from Mythic
+    total: int
+    page: int
+    page_size: int
+
+
+def decode_response_text(raw: str) -> str:
+    """Decode a base64-encoded Mythic response_text field.
+
+    On binascii.Error (binary payload) returns "<binary> " + hex string
+    so execution_result never silently corrupts.
+    """
+    try:
+        return base64.b64decode(raw).decode("utf-8")
+    except binascii.Error:
+        return "<binary> " + raw.encode().hex()
+    except UnicodeDecodeError:
+        raw_bytes = base64.b64decode(raw)
+        return "<binary> " + raw_bytes.hex()
+
+
+class C2Adapter(ABC):
+    """Thin interface over a C2 backend (Mythic or custom)."""
+
+    @abstractmethod
+    def test_connection(self) -> C2Health:
+        """Verify that the C2 is reachable and the token is valid."""
+        ...
+
+    @abstractmethod
+    def list_callbacks(self) -> list[C2Callback]:
+        """Return active callbacks visible to this API token."""
+        ...
+
+    @abstractmethod
+    def create_task(
+        self,
+        callback_display_id: int,
+        command: str,
+        params: str | None = None,
+    ) -> int:
+        """Issue a task and return its Mythic display_id."""
+        ...
+
+    @abstractmethod
+    def get_task(self, task_display_id: int) -> C2TaskStatus:
+        """Return current status of a task."""
+        ...
+
+    @abstractmethod
+    def get_task_output(self, task_display_id: int) -> str:
+        """Return decoded, concatenated output for a completed task."""
+        ...
+
+    @abstractmethod
+    def list_callback_tasks(
+        self,
+        callback_display_id: int,
+        page: int = 1,
+        page_size: int = 25,
+    ) -> C2TaskPage:
+        """Return a paginated history of tasks for a callback."""
+        ...