Implement advanced mail features (forward, receipts, corp broadcast, credits, quotas, purge) #442
Description
Add the missing mail system features required for canonical parity.
Parent: #437
Summary:
- Forward messages to other players.
- Support delivery/read receipts.
- Corp-wide broadcast mail.
- Attach credits (donations) to messages.
- Enforce mailbox quotas and provide warnings.
- Tools to purge old messages.
Acceptance criteria:
- Schema updated to store receipts, attachments, and quotas.
- New protocol commands for the above operations.
- Admin tools/flags for moderation and debugging.
See epic for canon doc links.