refactor: improve code readability and structure in various components

- Refactored the `mini_app_short_name` assignment in `config.py` for better clarity.
- Enhanced the `app_config_js` function in `app.py` to improve formatting of the JavaScript response body.
- Added per-chat locks in `group_duty_pin.py` to prevent concurrent refreshes, improving message handling.
- Updated `_schedule_next_update` to include optional jitter for scheduling, enhancing performance during high-load scenarios.
- Cleaned up test files by removing unused imports and improving test descriptions for clarity.

tests/test_app.py

@@ -1,7 +1,6 @@
 """Tests for FastAPI app /api/duties."""
 
 import time
-from pathlib import Path
 from unittest.mock import ANY, patch
 
 import pytest