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/schemas.py

@@ -91,6 +91,7 @@ class EntityCreate(BaseModel):
     type: EntityType
     vintage_year: int | None = None
     fund_size_cents: int | None = None
+    linked_user_id: int | None = None
 
 
 class EntityUpdate(BaseModel):
@@ -99,6 +100,7 @@ class EntityUpdate(BaseModel):
     vintage_year: int | None = None
     fund_size_cents: int | None = None
     status: EntityStatus | None = None
+    linked_user_id: int | None = None
 
 
 class EntityResponse(BaseModel):
@@ -108,6 +110,7 @@ class EntityResponse(BaseModel):
     vintage_year: int | None
     fund_size_cents: int | None
     status: EntityStatus
+    linked_user_id: int | None = None
     created_at: datetime