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/domain/models.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 from dataclasses import dataclass, field
 from datetime import datetime
 from enum import Enum
-from typing import Protocol, runtime_checkable
+from typing import Any, Protocol, runtime_checkable
 
 
 class MarketCode(str, Enum):
@@ -116,6 +116,9 @@ class Portfolio:
     price: float = 0.0
     profit_buffer: float = 0.0
     active_strategy_id: str | None = None
+    strategy_name: str | None = None  # REQ-055: strategy_sets.name JOIN 결과
+    strategy_params: dict[str, Any] = field(default_factory=dict)  # REQ-055: strategy_sets.config 파싱 결과
+    markets: list[str] = field(default_factory=list)  # REQ-055: assets.markets JOIN 결과
     avabile_strategies: list[str] | None = None
     created_at: datetime | None = None
     updated_at: datetime | None = None