refactor: CB 게이지 pnl_pct 저장 레이어를 L7 → L6_DAILY로 변경

portfolio_pnl_pct는 일별 성과 지표이므로 실시간 종목 데이터(L7)보다
일별 P&L 레이어(L6_DAILY)가 더 적합함. (PR #197 코드리뷰 반영)

- main.py: L7_REALTIME + ISO timestamp → L6_DAILY + date(YYYY-MM-DD)
- app.py: contexts 쿼리 layer/timeframe 조건 동기화
- tests: _seed_cb_context L6_DAILY + today 날짜로 수정

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_dashboard.py

@@ -355,18 +355,20 @@ def test_positions_empty_when_no_trades(tmp_path: Path) -> None:
 
 def _seed_cb_context(conn: sqlite3.Connection, pnl_pct: float, market: str = "KR") -> None:
     import json as _json
+    from datetime import UTC, datetime
+    today = datetime.now(UTC).date().isoformat()
     conn.execute(
         """
         INSERT OR REPLACE INTO contexts (layer, timeframe, key, value, created_at, updated_at)
         VALUES (?, ?, ?, ?, ?, ?)
         """,
         (
-            "L7_REALTIME",
-            "2026-02-21T10:00:00+00:00",
+            "L6_DAILY",
+            today,
             f"portfolio_pnl_pct_{market}",
             _json.dumps({"pnl_pct": pnl_pct}),
-            "2026-02-21T10:00:00+00:00",
-            "2026-02-21T10:00:00+00:00",
+            f"{today}T10:00:00+00:00",
+            f"{today}T10:00:00+00:00",
         ),
     )
     conn.commit()