Historic Data for pgexporter

This project will add native historical metrics storage to pgexporter, enabling users to analyze trends and long-term performance without relying on external time-series stacks. The solution introduces an optional history module, configurable via the main settings, that can store metrics in SQLite, PostgreSQL, or TimescaleDB. Deliverables include the history storage module, a REST API for querying historical data, console integration for visualizing trends, migration/export tooling, and clear documentation for setup and backend selection.