PAPI Support — Expose Placeholders for Firm Balance

Add PlaceholderAPI support exposing firm details — notably firm balance — as placeholders.

Technical notes — No PlaceholderAPI expansion exists anywhere in the economy plugins. Firm balance is computed internally by FirmTransactionServiceImpl.getAggregateBalance (sums treasury.getBalanceByAccountId over FirmAccountsMapper.listAccountsByFirm). Add a PlaceholderExpansion (soft-depend PAPI in plugin.yml, register in onEnable) backed by the public BusinessApi, wiring %business_firm_balance_<name>%. Blocked on PAR-22 — needs a public FirmApi.getTotalBalance first.

Implementation

PR: https://github.com/ParadauxIO/hibernia-economy/pull/17 (targets develop).

Adds the optional bundled business PlaceholderAPI expansion, public-API balance lookup, reload persistence and disable cleanup, documentation, and tests for absent PlaceholderAPI in CI.

Validation: bash gradlew :business:build -Pci=true --console=plain passed: 574 unit/integration tests, 4 tests without PlaceholderAPI, 4 without CarbonChat, shaded plugin build, and 96.3% line coverage (95% gate).