feat: implement complete CMT backend with API endpoints and test suite

- Add 7 core API endpoints: users, transactions, partners, products, inventory, payments, credit
- Implement role-based authentication (admin/write/read-only access)
- Add comprehensive database models with proper relationships
- Include full test coverage for all endpoints and business logic
- Set up Alembic migrations and Docker configuration
- Configure FastAPI app with CORS and database integration

backend/tests/test_main.py

@@ -0,0 +1,5 @@
+def test_read_root(client):
+    """Test the root endpoint."""
+    response = client.get("/")
+    assert response.status_code == 200
+    assert response.json() == {"message": "CMT API v1"}