Read the caller's org mnemom-native monthly usage statement

The mnemom-NATIVE monthly usage statement for the authenticated caller's organization — the itemized statement mnemom itself produces from its OWN ledger (mu_view_statement_lines), independent of Stripe's hosted invoice PDF. This is what backs the first-party downloadable PDF/CSV: a customer can obtain a record of their metered usage even when they have no Stripe invoices at all (the common prepay case). Same org-context + RBAC discipline as GET /billing/mu: RBAC is enforced before any read, and the account id is the SERVER-RESOLVED billing_account_id, never client-supplied. period is a required YYYY-MM month. READ-ONLY by construction: api presents its own mu:check service key, which carries no spend or grant scope, and the statement is a pure projection of already-recorded usage — nothing is minted or mutated. Forwards to the frozen mnemom-billing upstream GET /v1/accounts/{accountId}/statement; the {accountId, period, lines, totalUsd} body is passed through verbatim. 503 when the mnemom-mu binding/URL is unconfigured (fail-closed).