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/__init__.py

@@ -6,7 +6,7 @@ from pathlib import Path
 
 from flask import Flask, jsonify, send_from_directory
 
-from backend.app.api import auth_bp, engagements_bp, simulations_bp, templates_bp, users_bp
+from backend.app.api import auth_bp, c2_bp, engagements_bp, simulations_bp, templates_bp, users_bp
 from backend.app.cli import register_cli
 from backend.app.config import Config, TestConfig
 from backend.app.errors import register_error_handlers
@@ -38,6 +38,7 @@ def create_app(config_object: object | None = None) -> Flask:
     app.register_blueprint(engagements_bp)
     app.register_blueprint(simulations_bp)
     app.register_blueprint(templates_bp)
+    app.register_blueprint(c2_bp)
 
     from backend.app.services import mitre as mitre_svc
     mitre_svc.load_bundle()