Add /firm baltop — leaderboard of firms by collective account balance

Add a /firm baltop [page] command that ranks active firms by the collective balance of every treasury account the firm owns, mirroring Treasury's /baltop. Output like 1. Acme $1,234.00.

Design

• Aggregate in FirmTransactionService via TreasuryApi.getBalancesByIds(...) — business must not read treasury tables directly.
• New FirmAccountsMapper.listActiveAccountLinks() returning every (firm_id, account_id) with removed_at IS NULL (one query, no N+1).
• New FirmBalanceEntry model (firmId, displayName, balance), returned as Page<FirmBalanceEntry>.
• FirmCommands routes baltop / baltop <page>, perm business.baltop (default true), paginated 10/page.
• Messages: header/entry/footer/empty. Permission node in plugin.yml.
• Unit tests for the new service method (95% coverage gate).