Abuse and rate limiting on the public claim endpoints

## Question

What protects the unauthenticated, mutating claim endpoints from abuse?

Graduated from the map's fog now that [Data model, schema, and route and permission map](https://github.com/fkeenv/tabby/issues/11) has drawn the route family it was waiting on. Eight routes under `/g/{claimToken}` accept writes from anybody holding a 22-character string, and [ADR-0003](https://github.com/fkeenv/tabby/blob/main/docs/adr/0003-the-claim-link-has-no-identity-boundary.md) makes that deliberate — "the link is the entire attack surface, so its hygiene is the whole defence". The hygiene list it names is partly satisfied by the route map; the rate limiting on it is not, because the shape of the protection was unknowable before the endpoints existed.

- **Token guessing.** 128 bits makes a hit implausible, but an unthrottled endpoint still invites the attempt, and every miss is a database lookup. Whether unknown-token 404s are throttled separately from valid-token traffic — and per IP, since there is nothing else to key on — is the first question.
- **What the throttle key is on a valid token.** Per IP punishes exactly the case the product is designed for: one restaurant table on one wifi, or five phones behind one carrier NAT. Per token punishes the whole table for one bad actor. Per token *and* IP is a third option with different failure modes. There is no session and no account to key on ([ADR-0003](https://github.com/fkeenv/tabby/blob/main/docs/adr/0003-the-claim-link-has-no-identity-boundary.md)), which removes the usual answer.
- **The claim endpoint is chatty by design.** Tap a tile, take a Share — the winning prototype's whole feel depends on that being instant and repeatable, so a limit tuned for a form post will make the product feel broken. What a legitimate burst looks like needs a number.
- **Participant creation is the dangerous one.** [Roster repair](https://github.com/fkeenv/tabby/issues/14) already observes that conjuring a Participant is the strictly more dangerous act than renaming one, and merge and delete are the Organizer's alone — so a flood of junk Participants is repair work that only one person can do. Whether that endpoint gets a tighter limit than claiming is a real choice.
- **What a throttled response says.** A 429 on a screen with no accounts and no support channel has to say something a stranger in a restaurant can act on.
- **Where the limit lives.** Laravel's `throttle` middleware with a named limiter is the default and needs no dependency; a CDN or edge rule is a deployment answer that would leave the app unprotected in local and CI. Pick one and say why.
- **Not in question:** the Claim Link's five powers, and the accepted permanent risk that a holder can claim as somebody else. This ticket is about volume, not about identity.

Blocks the spec.