perf(intelligent-eval): 修复 N+1 查询和参数名混淆

- expire_stale_running_sessions: 使用单次 JOIN 查询替代 N+1 查询
  将每个会话单独查询最后消息时间改为一次性获取所有 running 会话及其最后消息时间

- submit_report/evals_needing_analyst_nudge: 消除 session/sessions 参数名混淆
  将局部变量 sessions 重命名为 eval_sessions，避免与数据库会话参数 session 混淆

这些改进提升了查询性能并增强了代码可读性。

backend/agenteval/intelligent_eval/lifecycle.py

@@ -226,8 +226,8 @@ def submit_report(session: Session, eval_id: str, report: dict[str, Any]) -> Int
     （不完整证据），不再阻塞报告提交。
     """
     repo = IntelligentEvalRepository(session)
-    sessions = IntelligentEvalSessionRepository(session).list_by_eval(eval_id)
-    if any(s.status == IntelligentEvalSessionStatus.RUNNING for s in sessions):
+    eval_sessions = IntelligentEvalSessionRepository(session).list_by_eval(eval_id)
+    if any(s.status == IntelligentEvalSessionStatus.RUNNING for s in eval_sessions):
         raise IntelligentEvalTransitionError("存在进行中的会话，不能提交报告")
     # ADR-0011：submit 边界把 scores 归一到 {overall, dimensions} 单一规范结构
     if report.get("scores"):
@@ -432,29 +432,36 @@ def expire_stale_running_sessions(session: Session) -> int:
     now = utc_now()
     # SQLite 读出为 naive datetime，阈值须同为 naive 才能在 Python 侧比较
     threshold = now.replace(tzinfo=None) - timedelta(minutes=SESSION_IDLE_EXPIRE_MINUTES)
-    rows = session.exec(
-        select(IntelligentEvalSessionDB).where(IntelligentEvalSessionDB.status == "running")
-    ).all()
+    
+    # 单次查询获取所有 running 会话及其最后消息时间（避免 N+1 查询）
+    stmt = (
+        select(
+            IntelligentEvalSessionDB,
+            func.max(IntelligentEvalMessageDB.created_at).label("last_message_at"),
+        )
+        .outerjoin(
+            IntelligentEvalMessageDB,
+            IntelligentEvalMessageDB.session_id == IntelligentEvalSessionDB.id,
+        )
+        .where(IntelligentEvalSessionDB.status == "running")
+        .group_by(IntelligentEvalSessionDB.id)
+    )
+    rows = session.exec(stmt).all()
 
     expired = 0
-    for row in rows:
-        last_message_at = session.exec(
-            select(func.max(IntelligentEvalMessageDB.created_at)).where(
-                IntelligentEvalMessageDB.session_id == row.id
-            )
-        ).one()
-        last_activity = last_message_at or row.created_at
+    for session_row, last_message_at in rows:
+        last_activity = last_message_at or session_row.created_at
         if last_activity is None or last_activity >= threshold:
             continue
-        row.status = IntelligentEvalSessionStatus.EXPIRED.value
-        row.closed_at = now
+        session_row.status = IntelligentEvalSessionStatus.EXPIRED.value
+        session_row.closed_at = now
         expired += 1
         append_decision_log(
-            row.eval_id,
+            session_row.eval_id,
             "session_expired",
             f"平台兜底：会话 {SESSION_IDLE_EXPIRE_MINUTES} 分钟无新轮次，置为过期（不完整证据）",
             "platform",
-            {"platform_supplemented": True, "session_id": row.id, "turn_count": row.turn_count},
+            {"platform_supplemented": True, "session_id": session_row.id, "turn_count": session_row.turn_count},
             session,
         )
 
@@ -674,16 +681,16 @@ def evals_needing_analyst_nudge(session: Session) -> list[str]:
 
     needing: list[str] = []
     for row in executing:
-        sessions = session.exec(
+        eval_sessions = session.exec(
             select(IntelligentEvalSessionDB).where(IntelligentEvalSessionDB.eval_id == row.id)
         ).all()
-        if not sessions or any(s.status not in _TERMINAL_SESSION_STATUSES for s in sessions):
+        if not eval_sessions or any(s.status not in _TERMINAL_SESSION_STATUSES for s in eval_sessions):
             continue
         # 冒烟教训：窗口未结束且会话数未达计划时，未来时段到期后还要建会话，
         # 此时催促 analyst 会让报告提前收敛（漏掉后续时段的证据）
-        if _window_has_pending_future_slots(row, sessions, now):
+        if _window_has_pending_future_slots(row, eval_sessions, now):
             continue
-        closed_moments = [s.closed_at for s in sessions if s.closed_at is not None]
+        closed_moments = [s.closed_at for s in eval_sessions if s.closed_at is not None]
         if not closed_moments:
             continue
         last_closed = max(closed_moments)