0.2.23: link GP entities to their investor account for auto asset balances

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.

deploy/package.json

@@ -1,6 +1,6 @@
 {
   "name": "ten31portal-startos",
-  "version": "0.2.22",
+  "version": "0.2.23",
   "private": true,
   "scripts": {
     "build": "npm run check && rm -rf ./javascript && ncc build startos/index.ts -o ./javascript",