Improve database outage recovery and player data cleanup

[Minecraft0122]

Summary

• retain queued database transactions during transient/recoverable connection outages and retry them in order with bounded exponential backoff
• add /plan db remove_registered <after-date> <before-date> for confirmed, inclusive bulk deletion by first-join date
• add regression coverage for merging a SQLite database into a non-empty MySQL-compatible destination while preserving existing server data
• fix transaction queue accounting when throwaway transactions are dropped

Verification

• GitHub Actions full CI passed: https://github.com/Minecraft0122/Plan/actions/runs/30552479680
• full Gradle build and test suite, including MariaDB/MySQL and SQLite tests
• Selenium/frontend build and Javadoc generation
• local Checkstyle and focused database/command regression tests

The SQLite-to-MySQL merge command already exists as /plan db merge SQLite MySQL; this PR adds the non-empty destination regression test requested for that workflow.

Closes #4727
Closes #4719