refactor: CB 게이지 저장소를 context tree → system_metrics 별도 테이블로 분리

대시보드 표시 전용 데이터를 AI 의사결정용 context tree에 저장하는 것은
관심사 분리 위반. system_metrics 경량 테이블을 신설하여 완전히 분리. (PR #197 코드리뷰 반영)

- db.py: system_metrics 테이블 추가 (key/value/updated_at)
- main.py: context_store.set_context(L6_DAILY) → db_conn.execute(system_metrics)
- app.py: contexts 쿼리 → system_metrics 쿼리
- tests: _seed_cb_context를 system_metrics 삽입으로 변경

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

tests/test_dashboard.py

@@ -355,20 +355,12 @@ 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 (?, ?, ?, ?, ?, ?)
-        """,
+        "INSERT OR REPLACE INTO system_metrics (key, value, updated_at) VALUES (?, ?, ?)",
         (
-            "L6_DAILY",
-            today,
             f"portfolio_pnl_pct_{market}",
             _json.dumps({"pnl_pct": pnl_pct}),
-            f"{today}T10:00:00+00:00",
-            f"{today}T10:00:00+00:00",
+            "2026-02-22T10:00:00+00:00",
         ),
     )
     conn.commit()