fix(node): chat is bounded in size and in rate

A chat message is the plainest member-supplied write there is: the node stores
it in `chat.db`, where nothing expires it — retention is a manual command
(§6.6) — relays it to every other connected member, and has the hub write a
notification for every member of the group. Nothing bounded any of it. The only
ceiling was the DataChannel frame, 64 MB once the handshake is done, so one
member in a loop filled the operator's disk and saturated everyone else's
connection, and the node's answer to each message was `ack`.

Uploads, the other member-supplied write, have carried a filename allowlist,
strict chunk ordering, a no-overwrite rule and a 4 GB cap since C5a — because
somebody asked what one member costs the others on that path. Nobody had asked
it on this one.

Two bounds, for the two halves of the question: **64 KB of ciphertext** for what
one message may cost, and **60 a minute per account per group** for how often
one member may impose it. Both are checked before anything is stored or relayed;
a refusal names itself and is audited, so "why is my disk full" has an answer.
The rate is keyed by account, not by connection: a second tab does not make
anyone type faster, and keying on the session would hand a script one budget per
socket it opens.

No node-wide ceiling beside it, deliberately. The link-preview limiter has one
because a preview spends the *node's* egress and its third-party quota, which is
one shared thing; a chat message spends the sender's own group, and a node-wide
ceiling would let a busy group silence a quiet one — this same defect one level
up. The last test in the new file is that property: a member at their limit has
not spent anybody else's.

Two things stay open on purpose and are named rather than quietly done:
retention still keeps everything, because a default that deletes people's
history is not a review's call; and the composer still offers to send an
oversized message, so this is §6.4's pattern with only the node half built.

§6.6 gains the rule, §13.5b the label — AV20, with AV21–AV23 registering the
three fixes this week that closed the same kind of gap elsewhere.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UMxEQadpzPkYLFf5CYKhpW

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