chore: update project documentation and configuration files

- Added AGENTS.md for AI agent documentation and maintainers, outlining project structure and conventions.
- Updated CONTRIBUTING.md to specify that all project documentation must be in English, including README and docstrings.
- Enhanced README.md to reference documentation guidelines and the new AGENTS.md file.
- Cleaned up .gitignore by removing unnecessary entries for cursor-related files.
- Introduced new .cursor rules for backend, frontend, project architecture, and testing to standardize development practices.

README.md

@@ -151,6 +151,7 @@ Tests cover `api/telegram_auth` (validate_init_data, auth_date expiry), `config`
 - **Commits:** Use [Conventional Commits](https://www.conventionalcommits.org/): `feat:`, `fix:`, `docs:`, `refactor:`, `test:`, `chore:`, etc.
 - **Branches:** Follow [Gitea Flow](https://docs.gitea.io/en-us/workflow-branching/): main branch `main`, features and fixes in separate branches.
 - **Changes:** Via **Pull Request** in Gitea; run linters and tests (`ruff check .`, `pytest`) before merge.
+- **Documentation:** Project documentation is in English; see [CONTRIBUTING.md](CONTRIBUTING.md) for details.
 
 ## Logs and rotation