Group inventory: add atomic group-to-trainer transfer route

[prodmodfour]

Goal

Move item quantities from the shared group inventory into a trainer sheet atomically.

Scope

• Add a use case and API route for group-to-trainer transfers.
• Accept groupSlug, groupRevision, trainerSlug, trainerRevision, section, itemId, and quantity.
• In one SQLite transaction:
  • read the group inventory and trainer sheet;
  • validate both revisions;
  • decrement/remove the group row;
  • merge the item into the trainer inventory section;
  • increment both document revisions;
  • return the authoritative group inventory and trainer sheet update.
• Start with GM-only access in this ticket.

Acceptance criteria

• Successful transfer updates both documents atomically.
• If either revision is stale, neither document changes.
• Invalid quantity, missing row, missing trainer, and equipment partial-transfer cases are rejected clearly.
• Tests prove rollback when the second write fails.

Depends on

• Group inventory: add SQLite repository with revision-checked updates #29
• Group inventory: add shared item merge and quantity transfer helpers #37