Group inventory: document workflow and future live-play command boundary

[prodmodfour]

Goal

Document how group inventory works now and where live-play command integration should happen later.

Scope

• Add or update docs explaining:
  • group inventory is campaign-level SQLite state, not map metadata or a fake trainer sheet;
  • GM direct edits are revision-checked saves;
  • player transfers are limited to linked trainer sheets;
  • realtime updates keep other open clients in sync;
  • live-play command scopes are intentionally deferred unless a later feature needs in-map item consumption.
• Mention any production/auth caveats consistent with the existing trusted-table model.

Acceptance criteria

• Documentation is linked from an appropriate architecture or feature doc.
• The doc gives future implementers a clear boundary for possible groupInventory live-play scopes without requiring that work now.
• No code behavior changes are added in this ticket.

Depends on

• Group inventory: publish realtime updates for saves and transfers #42
• Group inventory: include campaign inventories in maintenance export/backup scripts #43