feat(storage): add local Postgres runtime and migrations

[kpeez]

This pull request introduces significant improvements to the project's development workflow, backend infrastructure, and documentation. The main changes include the addition of a comprehensive CI workflow, the introduction of a backend database schema and migration system using Alembic, and updates to documentation to better guide local development. There are also adjustments to backend configuration and pre-commit hooks to improve developer experience and consistency.

Continuous Integration and Developer Workflow:

• Added a new GitHub Actions workflow (.github/workflows/ci.yml) that automates linting, building, type checking, and testing for both frontend and backend, including setup for Postgres and dependency management.
• Updated pre-commit hooks to use local ruff and ruff format commands via uv, ensuring consistency with local development environments.

Backend Infrastructure and Schema Management:

• Introduced Alembic for backend database migrations, including configuration files (alembic.ini, env.py, script.py.mako) and an initial migration defining tables for documents, document chunks, conversations, messages, and app state. [1] [2] [3] [4]
• Added backend configuration utilities for environment-based database connection management in paperchat_backend/config.py.

Documentation and Project Setup:

• Rewrote README.md to focus on local development, providing clear instructions for setting up and verifying both backend and frontend environments.
• Removed the .python-version file to avoid confusion about the Python version requirement.
• Added a __version__ attribute to the backend package for version tracking.

Other Notable Changes:

• Removed the legacy app.py Streamlit application, reflecting the shift to a new backend architecture.
• Updated backend API bootstrap logic to use a new service-based approach for health checks. [1] [2]
• Removed the LICENSE file (possibly to be replaced or updated in a future commit).

These changes collectively modernize the project's backend, streamline development workflows, and clarify setup processes for contributors.