docs(issues): close epic 1525 persistence overhaul

Close EPIC torrust#1525 by moving the EPIC issue document and the final PostgreSQL sub-issue document to docs/issues/closed after finishing the incremental implementation plan.

History:
- The EPIC started months ago when we identified that adding PostgreSQL support was not straightforward with the previous persistence crate and required migration to sqlx.
- DamnCrab opened PR torrust#1684, followed by PR torrust#1695 after review feedback.
- Additional reviewer guidance was provided through PR torrust#1700 to help address change requests.
- The final implementation was completed through the incremental plan documented in issue comment #4294475557 on issue torrust#1525.
- The tracker now supports PostgreSQL.

Ideas initially introduced in DamnCrab's PRs and retained in the final implementation:
- Matrix DB compatibility script to validate tracker compatibility across database versions.
- End-to-end tracker tests using a real BitTorrent client (containerized qBittorrent).
- Basic database benchmarking to compare persistence performance before/after migration to sqlx and across database engines.

Thanks to DamnCrab for valuable contributions and ideas that improved the final result.

Co-authored-by: DamnCrab <42539593+DamnCrab@users.noreply.github.com>

Author: josecelano