Prune destroyed ships safely

[voidhelm-labs26]

Restores destroyed-ship filtering in leaderboard wealth and adds safe retention pruning for aged destroyed ship rows.

• rebuilds leaderboard_wealth to exclude destroyed ships from personal and corporation aggregates
• adds prune_destroyed_ships() with daily pg_cron scheduling
• scrubs historical event and port transaction references before deleting aged ship rows
• adds regression coverage for pruning and leaderboard behavior
• Refs voidhelm-labs26/voidhelm#15