feat(hub): the mail state is a panel section, and a ceiling falling is an event

The figure was a line beside the settings form, which is where it is changed
and not where it is watched. It sits with the other live figures under
Statistics now — four cards and, above them, a banner saying which of the two
ceilings has fallen. The two states are not the same to whoever is reading:
one means newcomers are turned away, the other means somebody locked out of
their account cannot get back in. The settings block keeps a line pointing at
it.

And an operator no longer has to be looking. When a global ceiling is reached
the administrators are notified — in `mail.py`, in its own session, never
raising, because this runs while a request is being refused and an alert that
fails must not turn a refusal into a 500. Once per hour, keyed on a row
rather than a flag in memory: a flood is what spends the budget, so one alert
per refusal would bury the message under its own cause, and a hub that is
refusing mail is a hub somebody is about to restart.

`/v1/admin/mail` gains `general_exhausted` and `all_exhausted` rather than
leaving the panel to compare two numbers.

Labels in all ten catalogues; `.warn-msg` for the middle state, on the
`--warn` token both themes already define.

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

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