fix(hub): the mail server is not a relay

The previous commit metered the paths that send mail. It was not enough, and
saying it was would have been wrong: a 60-second cooldown per account still
allows one stranger a minute — 1440 a day — and registration is open, so
"per account" is a bound an attacker buys more of. And there was a third door
nobody had counted.

  POST  /v1/users/register            an address nobody has verified
  PATCH /v1/users/me                  an address nobody has verified, signed in
  POST  /v1/users/password/reset      only the address already on file
  POST  /v1/groups/{id}/invite-notify only a registered member's address

The widest was the register *resend* branch: no token, no captcha, and the
username and address are the caller's own from a moment ago — registering a
victim's address once bought the right to mail them at the endpoint's rate
limit for as long as the account stayed pending.

So the bound moves into `mail.py`, where every message passes one function.
`purpose` is keyword-required and checked against a closed list, so a helper
that names anything else does not send and one that names nothing is a
TypeError rather than an unrestricted send. Under it:

  - a bound per **recipient**, across every purpose, account and endpoint —
    what a person being mail-bombed actually experiences, and the only bound
    that describes it. Keyed on a hash, because this would otherwise be the
    one place in the hub holding plaintext addresses in memory (S2)
  - an instance-wide hourly ceiling, which cannot be bought with more accounts
  - a cooldown on the resend branch, a cooldown and a daily ceiling on the
    address change, and the IP-log entry that endpoint never wrote — alone
    among the ones that mail

The ceiling on address changes counts IP-log rows, not EmailVerification: the
handler deletes this account's unverified rows before writing a new one, so
counting those counts one, always. Which is what the first version of it did.

Refusals never carry the address: that line goes to the journal.

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

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