feat: Phase 8.5 — ConversationSummary 查询优化，支持 SQL 聚合获取消息统计

backend/internal/store/message_pg.go

@@ -118,3 +118,46 @@ func (r *PgMessageRepository) GetMessageCount(ctx context.Context, sessionID str
 	}
 	return count, nil
 }
+
+func (r *PgMessageRepository) GetSessionMessageStats(ctx context.Context, sessionIDs []string) (map[string]SessionMessageStats, error) {
+	if len(sessionIDs) == 0 {
+		return map[string]SessionMessageStats{}, nil
+	}
+
+	rows, err := r.pool.Query(ctx,
+		`WITH stats AS (
+			SELECT session_id, COUNT(*) AS cnt
+			FROM messages
+			WHERE session_id = ANY($1)
+			GROUP BY session_id
+		),
+		last_msg AS (
+			SELECT DISTINCT ON (session_id) session_id, content
+			FROM messages
+			WHERE session_id = ANY($1)
+			ORDER BY session_id, id DESC
+		)
+		SELECT s.session_id, s.cnt, COALESCE(lm.content, '')
+		FROM stats s
+		LEFT JOIN last_msg lm ON lm.session_id = s.session_id`,
+		sessionIDs,
+	)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	result := make(map[string]SessionMessageStats)
+	for rows.Next() {
+		var sid string
+		var stats SessionMessageStats
+		if err := rows.Scan(&sid, &stats.MessageCount, &stats.LastMessage); err != nil {
+			return nil, err
+		}
+		result[sid] = stats
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return result, nil
+}