feat: enhance error handling and configuration validation

- Added a global exception handler to log unhandled exceptions and return a generic 500 JSON response without exposing details to the client.
- Updated the configuration to validate the `DATABASE_URL` format, ensuring it starts with `sqlite://` or `postgresql://`, and log warnings for invalid formats.
- Introduced safe parsing for numeric environment variables (`HTTP_PORT`, `INIT_DATA_MAX_AGE_SECONDS`) with defaults on invalid values, including logging warnings for out-of-range values.
- Enhanced the duty schedule parser to enforce limits on the number of schedule rows and the length of full names and duty strings, raising appropriate errors when exceeded.
- Updated internationalization messages to include generic error responses for import failures and parsing issues, improving user experience.
- Added unit tests to verify the new error handling and configuration validation behaviors.

AGENTS.md

@@ -46,4 +46,6 @@ Docstrings and code comments must be in English (Google-style docstrings). UI st
 - **Branches:** Gitea Flow; changes via Pull Request.
 - **Testing:** pytest, 80% coverage target; unit and integration tests.
 - **Config:** Environment variables (e.g. `.env`); no hardcoded secrets.
+- **Database:** One logical transaction per `session_scope` — a single `commit` at the end of the business operation (e.g. in `run_import`). Repository helpers used inside such a flow (e.g. `get_or_create_user_by_full_name`) accept `commit=False` and let the caller commit once.
+- **Error handling:** Do not send `str(exception)` from parsers or DB to the user. Use generic i18n keys (e.g. `import.parse_error_generic`, `import.import_error_generic`) and log the full exception server-side.
 - **Cursor:** The project does not version `.cursor/`. You can mirror this file in `.cursor/rules/` locally; [AGENTS.md](AGENTS.md) is the single versioned reference for AI and maintainers.