Add support for E2E testing infrastructure and OAuth configurations

- Introduced make commands for E2E tests using Testcontainers and Schemathesis.
- Updated `.env.demo` with configurable OAuth settings for Google and GitHub.
- Enhanced `README.md` with updated environment setup instructions.
- Added E2E testing dependencies and markers in `pyproject.toml` for real PostgreSQL and API contract validation.
- Included new libraries (`arrow`, `attrs`, `docker`, etc.) for testing and schema validation workflows.

README.md

@@ -217,9 +217,8 @@ The fastest way to get started is with Docker:
 git clone https://github.com/yourusername/fast-next-template.git
 cd fast-next-template
 
-# Copy environment files
-cp backend/.env.example backend/.env
-cp frontend/.env.local.example frontend/.env.local
+# Copy environment file
+cp .env.template .env
 
 # Start all services (backend, frontend, database)
 docker-compose up