Group inventory: publish realtime updates for saves and transfers

[prodmodfour]

Goal

Keep other open clients in sync when the group inventory changes.

Scope

• Define a realtime channel/event type for group inventory updates, or extend the existing library mutation realtime pattern if that fits cleanly.
• Publish an authoritative group inventory update after GM save.
• Publish authoritative group inventory and affected trainer sheet updates after transfers.
• Include client IDs so the originating tab can ignore or reconcile its own echo consistently with existing realtime patterns.
• Keep this focused on realtime update publication/consumption, not live-play command scopes.

Acceptance criteria

• Two clients viewing the group inventory converge after a GM save.
• Two clients viewing the group inventory converge after a transfer.
• Originating client does not duplicate optimistic mutations.
• Tests cover event access/shape and client-side application where existing realtime test harnesses make that practical.

Depends on

• Group inventory: enable GM direct editing and save on the inventory page #36
• Group inventory: add transfer UI between party inventory and trainers #41