Get all master accounts with their slaves in a single call

Returns every account that currently has at least one slave copying from it, either through a direct account-to-account copier or through a strategy of that account. This is the bulk alternative to calling GET /accounts/{accountId}/slaves per account: each entry contains the master account, its slave accounts (same payload as the per-account endpoint) and the slave count. Only direct slaves are listed, there is no transitive expansion: in a chain A -> B -> C you get one entry for A with slave B and one entry for B with slave C, so an intermediate account appears both as a master and as a slave. Deleted master and slave accounts are excluded, so an empty result means no account is acting as a master.