Add registration-date player removal command

[Minecraft0122]

Summary

• add /plan db remove_registered <after-date> <before-date> with removeregistered and remove_between aliases
• parse inclusive yyyy-MM-dd boundaries using Plan's configured time zone
• require plan.data.clear and explicit confirmation before deleting matching players
• remove all Plan data through the existing per-player removal transaction and report the removed count

Tests

• SQLiteTest.removePlayersRegisteredBetweenDates() covers inclusive lower and upper boundaries and retains an out-of-range player
• PlanCommandTest verifies the command is registered and permission-protected

This is the player-cleanup portion of the closed #4772, split into a single-purpose PR as requested.

Closes #4719