feat(soju-operator): add migration module for cross-tenant user data transfer

Supports forward migration (shared -> dedicated bouncer), reverse
migration (dedicated -> shared, keeping one network), and user removal.
Copies all related data: networks, channels, delivery/read receipts,
and web push subscriptions.

Signed-off-by: Josephine Pfeiffer <hi@josie.lol>

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Josephine Pfeiffer <hi@josie.lol>
