Adds GET /admin/mailboxes — paginated platform-wide mailbox list that
joins email_accounts → users → organizations so the table answers
"whose mailbox is this and where does it live" without N+1 fetches.
Search covers mailbox email / owner email / org name; status filter
defaults to active so the active surface shows first ("inactive" /
"all" both available). Provider filter speeds up "show me every Gmail
mailbox" investigations. Cursor pagination matches the rest of the
admin lists.
Frontend page surfaces warmup-on/off, send budget, and last-sync time
with red-when-never / amber-when-stale-over-24h tone so an
investigator can spot dead mailboxes fast. Mailbox email links into
the owning user's detail page; org name links into the workspace
admin so the pivot path stays one click in either direction.
Gated on AdminPermViewUsers since mailbox triage is tightly coupled to
user/org context today; a dedicated bit can be carved later if
mailbox-specific actions land.
Matt