feat(portfolios): persist and validate daemon assignment on create/update

POST /v1/portfolios now forwards a non-null managed_by_daemon_id into
create persistence, and both create and PATCH apply the same daemon
validation contract (UUID format -> 422, missing/foreign -> 404,
inactive -> 409, admin target-owner basis). Daemon validation locks the
daemon row FOR SHARE inside the portfolio write transaction so
validation and write form one atomic decision boundary.

Adds domain errors, repository protocol/signature updates, API/PostgreSQL
tests (POST->GET persistence, no partial state on failure, assignment vs
deactivation race), and contract/data-model documentation.

tests/test_v1_portfolios_crud.py

@@ -97,6 +97,7 @@ class _FakeRepo:
         profit_buffer: float,
         active_strategy_id: str | None,
         avabile_strategies: list[str] | None,
+        managed_by_daemon_id: str | None = None,
     ) -> Portfolio:
         p = Portfolio(
             portfolio_id=portfolio_id,
@@ -111,6 +112,7 @@ class _FakeRepo:
             price=asset_price,
             profit_buffer=profit_buffer,
             tag_ids=[],
+            managed_by_daemon_id=managed_by_daemon_id,
         )
         self._portfolios[portfolio_id] = p
         return p