Document the `/vaults/{id}/accept` Endpoint, Including Its Token Requirement

[maugauwi-hash]

Labels: documentation, api-contract
Priority: High
Estimated Time: 30 minutes

Description:
shared/api-contract.md's endpoint table has no row for beneficiary acceptance at all, despite
both APIClient.acceptBeneficiary(vaultID:token:) (iOS) and ApiClient.acceptBeneficiary(vaultId)
(Android) implementing it — inconsistently, per #87/#109. The lack of documentation is very likely
why the two clients drifted in the first place.

Tasks:

• Add the POST /vaults/{id}/accept row to the endpoint table
• Document the token parameter (required per Formalize the Beneficiary-Acceptance Token Requirement (or Deliberately Drop It) #109's resolution) and its source (the invitation link's query string)
• Cross-reference from Add Token Verification to the Beneficiary Acceptance Flow #87/Formalize the Beneficiary-Acceptance Token Requirement (or Deliberately Drop It) #109 once resolved

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