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/models.py

@@ -87,6 +87,9 @@ class Entity(SQLModel, table=True):
     vintage_year: int | None = None
     fund_size_cents: int | None = None
     status: EntityStatus = Field(default=EntityStatus.active)
+    # For a GP/mgmt entity that is also an LP with capital accounts (e.g. Ten31 LLC), link to
+    # its investor account so its Assets view can pull real per-fund balances from the eNAV.
+    linked_user_id: int | None = Field(default=None, foreign_key="users.id", index=True)
     created_at: datetime = Field(default_factory=datetime.utcnow)