Claim an anonymous file into your account

Attaches the anonymous file a claimToken references (returned by anonymous uploads, Canva publishes, and the Canva design status endpoint) to the authenticated bearer user's account. The programmatic twin of the /api/claim browser link: use it when the claiming identity must be EXACTLY the connected account - e.g. the Canva panel claims into the account its OAuth grant belongs to in the same gesture as connecting. The file keeps its expiry and encryption key; claiming unlocks the owner surfaces (library, readership, settings). Idempotent for the same owner: re-claiming a file you already own returns 200. Scope files:write.