feat(req-055): expose strategy & market meta fields in PortfolioItem response (additive)

Adds 4 additive fields to GET /v1/portfolios (and single-portfolio) response:
- active_strategy_id (str|null): portfolios.active_strategy_id raw value
- strategy_name (str|null): strategy_sets.name via LEFT JOIN
- strategy_params (dict): strategy_sets.config JSON parsed; {} on missing/invalid
- markets (list[str]): assets.markets; [] on missing/empty

_SELECT_PORTFOLIO extended with LEFT JOIN strategy_sets; _row_to_portfolio
parses new columns (row[19-21]) with JSON error isolation (WARNING log, no 500).
_portfolio_to_item maps all 4 new fields. Existing 16 fields unchanged.
N+1 prevented: single SQL JOIN for both list and single-item paths.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VHtM5Ws1HKCKQ4JGi6GRb5

app/schemas/v1.py

@@ -181,6 +181,11 @@ class PortfolioItem(BaseModel):
     managed_by_daemon_id: str | None = None
     created_at: datetime | None = None
     updated_at: datetime | None = None
+    # REQ-055: 전략·시장 메타데이터 additive 필드
+    active_strategy_id: str | None = None
+    strategy_name: str | None = None
+    strategy_params: dict[str, Any] = Field(default_factory=dict)
+    markets: list[str] = Field(default_factory=list)
 
 
 class PortfolioListResponse(BaseModel):