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/api/v1_portfolios.py

@@ -81,6 +81,11 @@ def _portfolio_to_item(portfolio: Portfolio) -> PortfolioItem:
         managed_by_daemon_id=portfolio.managed_by_daemon_id,
         created_at=portfolio.created_at,
         updated_at=portfolio.updated_at,
+        # REQ-055: 전략·시장 메타데이터 additive 필드
+        active_strategy_id=portfolio.active_strategy_id,
+        strategy_name=portfolio.strategy_name,
+        strategy_params=portfolio.strategy_params,
+        markets=portfolio.markets,
     )