Ten31 LLC (and any GP/mgmt entity) can be linked to its investor account,
so its Assets tab shows its real capital-account balance in each fund,
pulled live from the eNAV capital accounts instead of manual entry.
- entities.linked_user_id (migration c9d0e1f2a3b4) + EntityCreate/Update/
Response fields; validated to be an investor account.
- Edit-entity form gains a "Linked investor account" picker for GP/mgmt.
- Assets tab now auto-lists the linked account's balance per fund (the
earlier manual-stakes API remains but is no longer used by the UI).
Verified: 13/13 backend tests pass; alembic head c9d0e1f2a3b4; frontend
tsc + vite build clean.
