Group inventory: allow player transfers for linked trainer sheets

[prodmodfour]

Goal

Extend transfer routes so player users can move items only for trainer sheets linked to their selected player profile.

Scope

• Update group-to-trainer and trainer-to-group transfer routes/use cases to accept player actors.
• For GM users, preserve unrestricted transfer authority.
• For player users:
  • require a selected player profile;
  • verify the target/source trainer sheet is linked to that profile;
  • reject transfers involving unlinked trainer sheets.
• Do not allow players to perform direct full-document group inventory saves.

Acceptance criteria

• Player with linked trainer can transfer group-to-trainer and trainer-to-group.
• Player without a selected profile is rejected with a clear message.
• Player with an unrelated profile/trainer link is rejected.
• GM behavior remains unchanged.
• Tests cover both transfer directions and both success/failure authorization paths.

Depends on

• Group inventory: add atomic group-to-trainer transfer route #38
• Group inventory: add atomic trainer-to-group transfer route #39