Mailbox listings grew unbounded as old sessions ended without
unregistering. This adds two layers of cleanup, configurable via
mailbox.json or `serve` flags:
- Lazy filter: list responses (REST /v1/list, MCP list_mailboxes)
drop mailboxes idle longer than hideAfterMinutes (default 24h),
while always keeping the caller and any sender with messages
pending for them.
- Background sweep: startServer runs an initial prune on boot and
schedules an unref'd interval timer that hard-deletes mailboxes
idle longer than deleteAfterMinutes (default 7d) which have no
pending messages, and wipes their delivered history.
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