feat(req-002): implement portfolio tagging and tag filters

app/domain/models.py

@@ -73,6 +73,7 @@ class Portfolio:
     status: str
     name: str
     symbol: str
+    tags: list[str] | None = None
     owner_user_id: str | None = None
     cash_balance: float = 0.0
     asset_quantity: float = 0.0
@@ -111,6 +112,13 @@ class PortfolioComment:
     updated_at: datetime | None = None
 
 
+@dataclass
+class PortfolioTag:
+    portfolio_id: str
+    tag: str
+    created_at: datetime | None = None
+
+
 @runtime_checkable
 class LiveAssetSource(Protocol):
     def get_asset_quantity(self, symbol: str) -> float: ...