Group inventory: add shared item merge and quantity transfer helpers

[prodmodfour]

Goal

Add pure helpers for moving item quantities between inventory documents before wiring server routes.

Scope

• Add utility functions for:
  • finding a group inventory row by section and row ID;
  • decrementing or removing a source row;
  • merging stackable rows by normalized item identity;
  • treating equipment rows as whole-row transfers;
  • moving money between documents if money transfer is included.
• Keep helpers pure and independent from SQLite/API code.
• Reuse trainer inventory section keys and entry types where possible.

Acceptance criteria

• Unit tests cover stackable item transfer, exact-quantity removal, partial decrement, equipment whole-row transfer, invalid quantities, missing rows, and name normalization.
• Helpers do not mutate their inputs unless explicitly documented and tested.
• No HTTP routes or UI are added in this ticket.

Depends on

• Group inventory: add shared document types and normalizers #27