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/models/c2_config.py

@@ -0,0 +1,34 @@
+"""C2Config model — per-engagement Mythic connection settings."""
+from __future__ import annotations
+
+from datetime import UTC, datetime
+
+from backend.app.extensions import db
+
+
+class C2Config(db.Model):  # type: ignore[name-defined]
+    __tablename__ = "c2_config"
+
+    id = db.Column(db.Integer, primary_key=True)
+    engagement_id = db.Column(
+        db.Integer,
+        db.ForeignKey("engagements.id", ondelete="CASCADE"),
+        nullable=False,
+        unique=True,
+        index=True,
+    )
+    url = db.Column(db.Text, nullable=False)
+    api_token_encrypted = db.Column(db.Text, nullable=False)
+    verify_tls = db.Column(db.Boolean, nullable=False, default=True)
+    created_at = db.Column(
+        db.DateTime, nullable=False, default=lambda: datetime.now(UTC)
+    )
+    updated_at = db.Column(db.DateTime, nullable=True)
+
+    engagement = db.relationship(
+        "Engagement",
+        backref=db.backref("c2_config", uselist=False, cascade="all, delete-orphan"),
+    )
+
+    def __repr__(self) -> str:
+        return f"<C2Config engagement_id={self.engagement_id}>"