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/alembic/versions/c9d0e1f2a3b4_entity_linked_user.py

@@ -0,0 +1,29 @@
+"""add entities.linked_user_id (link a GP entity to its investor account)
+
+Revision ID: c9d0e1f2a3b4
+Revises: b8c9d0e1f2a3
+Create Date: 2026-07-01 10:45:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision: str = 'c9d0e1f2a3b4'
+down_revision: Union[str, None] = 'b8c9d0e1f2a3'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    with op.batch_alter_table('entities', schema=None) as batch_op:
+        batch_op.add_column(sa.Column('linked_user_id', sa.Integer(), nullable=True))
+    op.create_index('ix_entities_linked_user_id', 'entities', ['linked_user_id'])
+
+
+def downgrade() -> None:
+    op.drop_index('ix_entities_linked_user_id', table_name='entities')
+    with op.batch_alter_table('entities', schema=None) as batch_op:
+        batch_op.drop_column('linked_user_id')