Add backend api and engine

api/.gitignore

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+# Python
+__pycache__/
+*.py[cod]
+*$py.class
+*.so
+.Python
+build/
+develop-eggs/
+dist/
+downloads/
+eggs/
+.eggs/
+lib/
+lib64/
+parts/
+sdist/
+var/
+wheels/
+*.egg-info/
+.installed.cfg
+*.egg
+
+# Virtual environments
+venv/
+ENV/
+env/
+.venv
+
+# Environment variables
+.env
+.env.local
+.env.*.local
+
+# Database
+*.db
+*.sqlite
+*.sqlite3
+
+# Vector store data
+data/vector_store/
+!data/vector_store/.gitkeep
+
+# IDE
+.vscode/
+.idea/
+*.swp
+*.swo
+*~
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Docker
+.docker/
+
+# Logs
+*.log
+
+# Pytest
+.pytest_cache/
+.coverage
+htmlcov/
+
+# mypy
+.mypy_cache/