Initialize Chart of Accounts

Create the graph's chart of accounts from a shipped template — the fresh-company path to native books. Use when the graph has NO chart (a QuickBooks-synced tenant never needs this: its chart arrives with the sync and stays after a sever) and before connecting a bank feed, which needs a chart to resolve against. Templates: saas (subscription software), services (professional services), product (inventory and COGS) — the chartTemplates GraphQL field lists them with names and account counts. Creates the chart, its coa_mapping structure and the template's CoA → rs-gaap mapping associations in one transaction, with the equity rows mapped by the entity's legal form (entity_type, defaulting to the entity's). Each entity of the group keeps its own chart: entity_id names a subsidiary, omitted means the group parent. One-time per entity: 409 once that entity has a chart — a chart is never replaced. Customize afterwards with update-taxonomy-block; accounts that carry activity are never deleted.

Idempotency: supply an Idempotency-Key header to make safe retries; replays within 24 hours return the same envelope. Reusing the key with a different body returns HTTP 409 Conflict.