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.

backend/ten31portal/routers/entity_router.py

@@ -174,12 +174,22 @@ def get_entity(
     return EntityResponse.model_validate(entity, from_attributes=True)
 
 
+def _validate_linked_user(linked_user_id: int | None, session: Session) -> None:
+    """A linked account (for a GP entity that is also an LP) must be an investor account."""
+    if linked_user_id is None:
+        return
+    linked = session.get(User, linked_user_id)
+    if linked is None or linked.role != UserRole.investor:
+        raise HTTPException(status_code=400, detail="Linked account must be an investor account.")
+
+
 @router.post("", status_code=201)
 def create_entity(
     body: EntityCreate,
     user: User = Depends(require_writer),
     session: Session = Depends(get_session),
 ) -> EntityResponse:
+    _validate_linked_user(body.linked_user_id, session)
     entity = Entity(**body.model_dump())
     session.add(entity)
     session.flush()
@@ -200,6 +210,8 @@ def update_entity(
     if entity is None:
         raise HTTPException(status_code=404, detail="Entity not found")
     changes = body.model_dump(exclude_unset=True)
+    if "linked_user_id" in changes:
+        _validate_linked_user(changes["linked_user_id"], session)
     for key, val in changes.items():
         setattr(entity, key, val)
     session.add(entity)