LuckPerms → explorer group reconciliation cron

Sync in-game LuckPerms membership into explorer access groups (consumes the group RBAC tables).

Done & pushed on develop (treasury-api-plugin 3d1c23c):

• LuckPerms optional softdepend, bound behind a Class.forName guard.
• GroupReconciliationTask (async, fixed interval): for each explorer_group with a luckperms_node, resolve members via UserManager.searchAll (offline-inclusive) and reconcile its source='luckperms' rows via ExplorerGroupMapper. Manual members untouched. Node is a LuckPerms group by default (doc/group.doc), else a permission node.
• ReconciliationConfiguration (enabled=false, interval-seconds=1800); scheduled in onEnable only when enabled and LuckPerms present.
• Pure ReconciliationDiff + JUnit5 test (plugin's first tests).

Remaining: enable + live-server verification with LuckPerms (cron is opt-in/off by default).