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/fake.py

@@ -0,0 +1,91 @@
+"""Deterministic in-memory C2 adapter — used when MIMIC_C2_ADAPTER=fake.
+
+Intended for integration tests and local development without a live Mythic instance.
+"""
+from __future__ import annotations
+
+from backend.app.services.c2.adapter import (
+    C2Adapter,
+    C2Callback,
+    C2Health,
+    C2TaskPage,
+    C2TaskStatus,
+)
+
+_FAKE_CALLBACKS = [
+    C2Callback(
+        display_id=1,
+        active=True,
+        host="WORKSTATION-01",
+        user="jdoe",
+        domain="LAB",
+        last_checkin="2026-06-10T00:00:00Z",
+    ),
+]
+
+_FAKE_TASKS: dict[int, dict] = {}
+_next_task_id = 100
+
+
+class FakeAdapter(C2Adapter):
+    """In-memory adapter with deterministic behaviour."""
+
+    def test_connection(self) -> C2Health:
+        return C2Health(ok=True)
+
+    def list_callbacks(self) -> list[C2Callback]:
+        return list(_FAKE_CALLBACKS)
+
+    def create_task(
+        self,
+        callback_display_id: int,
+        command: str,
+        params: str | None = None,
+    ) -> int:
+        global _next_task_id
+        tid = _next_task_id
+        _next_task_id += 1
+        _FAKE_TASKS[tid] = {
+            "display_id": tid,
+            "callback_display_id": callback_display_id,
+            "command": command,
+            "params": params,
+            "status": "submitted",
+            "completed": False,
+            "output": None,
+        }
+        return tid
+
+    def get_task(self, task_display_id: int) -> C2TaskStatus:
+        task = _FAKE_TASKS.get(task_display_id)
+        if task is None:
+            return C2TaskStatus(display_id=task_display_id, status="unknown", completed=False)
+        return C2TaskStatus(
+            display_id=task_display_id,
+            status=task["status"],
+            completed=task["completed"],
+        )
+
+    def get_task_output(self, task_display_id: int) -> str:
+        task = _FAKE_TASKS.get(task_display_id)
+        if task is None:
+            return ""
+        return task.get("output") or ""
+
+    def list_callback_tasks(
+        self,
+        callback_display_id: int,
+        page: int = 1,
+        page_size: int = 25,
+    ) -> C2TaskPage:
+        items = [
+            t for t in _FAKE_TASKS.values()
+            if t["callback_display_id"] == callback_display_id
+        ]
+        start = (page - 1) * page_size
+        return C2TaskPage(
+            items=items[start : start + page_size],
+            total=len(items),
+            page=page,
+            page_size=page_size,
+        )