Grant the caller's account its one-time starter μ balance, IF its contract is signed

Contract-gated, idempotent starter-balance grant (MNE-6427). api authenticates the caller (JWT), resolves their OWN org and billing account server-side (no client-supplied account/org/amount is honored — MNE-441 self-provisioning), and reads the contract gate from mnemom-billing's GET /v1/contracts (mu:check). If NO contract has signedOffAt set, returns 200 {granted:false, reason:"contract_not_signed"} and mints nothing. If a contract IS signed, forwards to billing's idempotent POST /v1/internal/accounts/{id}/intro-grant (mu:grant) and passes its result through verbatim — {granted:true, mmu} on first grant, {granted:false, reason:"already_granted"} on replay (billing dedupes on a key shared with retail card capture and personal-org signup, so the grant lands at most once per account). Fails CLOSED (503) whenever the gate cannot be verified — read/grant identity unset, or the contracts read errors — never a silent grant. "api never grants" (MNE-2516) holds by credential scope and a server-authored upstream body; api performs no ledger arithmetic and owns no amount.