fix(hub): no mail from the event loop, and a ceiling on every path that sends it

`users.py` and `admin.py` under the availability lens. `admin.py` needed
nothing — its moderator/admin line is drawn explicitly, self-modification is
refused, and every list it serves is bounded. `users.py` had four findings and
one of them is the worst of this whole pass.

  AV9   `mail._send` is `smtplib` with a ten-second timeout, called straight
        from four async handlers. That wait is not one request's, it is the
        instance's: nothing else served, no node socket read, no WebRTC offer
        relayed, until the MTA answers. Reachable by any signed-in user at
        request rate through the endpoint below. It has no symptom a test
        catches — everything simply works slowly, for everyone, whenever the
        mail server is having a bad day.

  AV10  `PATCH /v1/users/me` is the third path that makes the hub send mail
        and the only one with neither a rate limit nor a captcha, while
        `register` and `password/reset-request` have both. The address is any
        string the caller types and the duplicate check only rejects one
        already held by an account here, so every address *not* registered on
        this hub was a valid target: a relay for verification codes with the
        hub's own reputation attached. A rate limit counting by IP bounds a
        caller and not an inbox, so the floor under it is a cooldown per
        account — the same for a reset request, whose cost also lands in a
        mailbox that is not the asker's.

  AV11  `default_tab:` accepted any suffix on a `{key:path}` route with an
        unbounded Text value and no cap on rows: one account could write
        without limit into a table shared with everyone. The suffix is a group
        id, which is what the SPA writes, so it is checked as one. A key over
        64 characters was also a 500 rather than a 400 — the column is
        String(64), which PostgreSQL enforces and SQLite does not, so it would
        have appeared in production and in no test.

  AV12  `/v1/notifications` and `/v1/groups` had no upper bound on `limit` and
        no floor under `offset`, while every list in `admin.py` carries
        `le=200`. The group directory takes no authentication at all.

Two shapes recur and are now named in §13.5b: a limit written on one of
several equivalent paths, and a bound that counts the wrong thing.

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

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