feat(hub): the mail bounds are settings, with a panel to change them

They were constants in two modules, so an operator could not touch them
without editing code and redeploying — and the hour a budget runs out is not
when anyone wants to do that.

`[mail]` in hub.toml carries the defaults; the live values live in
`hub_settings`, read at each use. A missing row falls back to what the
configuration file says, so an instance that never opens the panel behaves as
its file describes. The panel sends only what changed, the hub clamps each
value to a stated range and refuses a key it does not know, and the response
is what gets rendered — so a clamped value is never shown as stored.

`GET /v1/admin/mail` is the other half. There was no way to see any of this:
a refusal was a line in the journal, so an instance that had stopped sending
sign-up codes looked, from the panel, exactly like one with no sign-ups. It
reports the hour's use, what is left for sign-ups, and what is left for
recovery — the difference between those two being the reserved share made
visible.

Labels in all ten catalogues.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T4YmK41VsEURWFdop4EEeT

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