fix: bound what one member can cost the others

An availability review, prompted by the group claim above: a participant
supplies input — who else bears the cost? Six answers where the cost fell on
someone other than the sender, and none of them needs an attacker.

  AV3  `chat_notify` carried a `group_id` the hub believed, so any connected
       node could write a notification to every member of any group on the
       hub, carrying a display string of its choosing, with its account
       having no relation to that group. This is the group claim again, two
       hundred lines further down the same socket. Gated on what the node is
       registered for, and metered: the fan-out is one write per member. The
       budget expires by time rather than on disconnect, or reconnecting
       would refill it and a node token is good for an hour.

  AV4  A swarm source named its own `endpoint` as free text documented as
       "ip:port", so an account could publish a third party's address — H6's
       `peer_ip` defect, never applied here. Nothing dials a swarm source
       today, which is the only reason it was not already a reflection
       primitive. It is a transport and a port now, never a host, and the
       number of hashes one account may claim is bounded: rows were keyed
       (hash, account) with no cap at all.

  AV5  `handle_webrtc_answer` resolved any pending `peer_id` from any node's
       socket. The answer is the SDP a browser then connects to. That this
       had not happened rested on a uuid4 being unguessable.

  AV6  `relay_register` had no authentication of any kind: it compared
       `pk_relay` against the approved value, which is a *public* key, so
       anyone who could read it could rewrite where the hub tells nodes to
       send relayed traffic. The module docstring promised signed JWTs and
       `jwt` was imported and never used.

  AV7  The node held unlimited peer connections and kept one that never
       completed a handshake for the life of the daemon. H6 bounded what one
       unauthenticated peer costs; the hub's cap is three offers in flight
       per *account*, a limit on each caller and not on the machine, so an
       operator's exposure grew with the size of their groups.

  AV8  `invite-notify` put a request-supplied `group_name` into the subject
       of an email the hub sends under its own domain, to any account, with
       no rate limit. The name comes from the group row now.

The tests are two accounts each, in one file that says why: a one-member test
proves a one-member property, and every finding here needed a second person
to exist at all. Each was checked against the unfixed code. Two did not
survive that check and were rewritten — one re-enacted the disconnect path
instead of running it (hence `forget_node`), the other called the reaper
itself and would have passed with the call removed from `handle_offer`.

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

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