In-game firm notification system

A reusable in-game notification primitive for firms, so the various "tell the firm something happened" features share one code path instead of each hand-rolling message.send(getOnlineEmployees(...), ...).

Goal — a FirmNotificationService (Guice singleton) with a small surface:

• notifyFirm(int firmId, String messageKey, Object... placeholders) → sends to all currently-online firm members (proprietor + current employees) via the existing FirmStaffService.getOnlineEmployees.
• notifyFirmExcept(int firmId, UUID actor, ...) → same, but skips the actor (so the person who triggered the event isn't notified of their own action).

Why a service, not inline sends — multiple features want this (incoming transfers, employee join/leave, promotion/demotion, proprietorship change, account changes). Centralising gives one place for: recipient resolution, an optional per-player mute/toggle later, and a consistent business.notify.* message namespace.

Technical notes — The recipient primitive already exists: FirmStaffService.getOnlineEmployees(firm) returns online staff + proprietor (used today by disband). Message.send(Collection<Player>, key, …) is already used. Add the service + impl under services/, bind in BusinessModule, add a business.notify.* message block. No Treasury changes — this only covers events Business itself originates; truly external incoming transfers remain blocked on PAR-75 (per-transfer event).

Consumers (built on this):

• PAR-56 — incoming-transfer notification (first consumer, shipped alongside this).
• Employee join / departure / promotion-demotion notifications (separate issues).

This release: ship the service + PAR-56 on top of it.