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/tests/test_entity_link.py

@@ -0,0 +1,31 @@
+"""Linking a GP entity to its investor account (so Assets can pull real balances)."""
+
+from tests.conftest import make_user
+from ten31portal.models import Entity, EntityType, UserRole
+
+
+def test_link_entity_to_investor(auth_client, session):
+    inv = make_user(session, username="ten31llc", role=UserRole.investor, name="Ten31 LLC")
+    gp = Entity(name="Ten31 LLC", type=EntityType.gp)
+    session.add(gp)
+    session.commit()
+    session.refresh(gp)
+
+    linked = auth_client.patch(f"/api/entities/{gp.id}", json={"linked_user_id": inv.id})
+    assert linked.status_code == 200, linked.text
+    assert linked.json()["linked_user_id"] == inv.id
+
+    # Unlink.
+    unlinked = auth_client.patch(f"/api/entities/{gp.id}", json={"linked_user_id": None})
+    assert unlinked.status_code == 200
+    assert unlinked.json()["linked_user_id"] is None
+
+
+def test_link_rejects_non_investor(auth_client, session):
+    staff = make_user(session, username="ops2", role=UserRole.operations)
+    gp = Entity(name="Mgmt Co", type=EntityType.mgmt_co)
+    session.add(gp)
+    session.commit()
+    session.refresh(gp)
+    resp = auth_client.patch(f"/api/entities/{gp.id}", json={"linked_user_id": staff.id})
+    assert resp.status_code == 400