Add docstrings to Settings property methods in config.py

[VenkataAnilKumar]

The five @property methods in src/backend/app/config.py (recommendations_table_name, approvals_table_name, action_history_table_name, kpi_table_name, confidence_threshold) expose public API used across routers but carry no docstrings.

Change: Add a one-line docstring to each property so IDEs surface inline hints and mypy/ruff checks are satisfied per the project coding conventions.

Risk: Zero — documentation only, no logic change.