feat(app): add the launcher-backed document library slice

[kpeez]

This pull request introduces several new features and improvements focused on enhancing local development experience, improving platform compatibility, and simplifying backend configuration. Notably, it adds a cross-platform native file picker for selecting PDF documents, streamlines the backend to use SQLite by default, and introduces a unified launcher workflow. There are also updates to documentation, CI workflows, and embedding service batching.

Major new features and improvements:

1. Cross-platform native file picker for PDFs

• Added a new backend service (local_files.py) that provides an API endpoint (/api/local-files/pick-documents) for picking PDF files using native OS dialogs on macOS, Windows, and Linux, with appropriate error handling if unavailable. [1] [2] [3]
• Introduced a response model for the file picker API.

2. Backend configuration and SQLite support

• Refactored backend configuration to use SQLite as the default database, with environment variable overrides (PAPERCHAT_DATABASE_URL, PAPERCHAT_DB_PATH). The backend now ensures the SQLite parent directory exists before migrations. [1] [2] [3] [4] [5] [6] [7] [8]
• Updated documentation to reflect the SQLite-based setup and removed references to Postgres.

3. Unified launcher and workflow improvements

• Added a new paperchat launch command and a corresponding CI workflow for streamlined local development and testing, including linting and formatting checks for the launcher. [1] [2] [3]
• Updated .pre-commit-config.yaml to include root-level Ruff checks and formatting for the launcher CLI and tests.

4. Embedding service batching and limits

• Improved the embeddings service to batch requests (batch size 32) and truncate text to a maximum length (4000 chars) for efficiency and reliability. [1] [2] [3]

5. Backend API and error handling enhancements

• Added a /api/runtime endpoint to provide runtime configuration details (version, paths, model info). [1] [2] [3] [4]
• Improved error handling in document import API to return a 400 error if the file is not found.

CI and workflow simplifications:

• Removed Postgres setup and teardown steps from CI, switching to SQLite for all backend jobs. [1] [2] [3]

These changes collectively make PaperChat easier to set up and use locally, improve platform compatibility, and streamline both development and CI workflows.