improvement(chat): 会话列表改用 session_last_message 缓存表

- 用 session.db::session_last_message 缓存会话最后一条消息预览，减少会话列表查询开销
- 增加缓存表构建/状态接口，列表缺失时自动补建
- 解密阶段支持自动构建，环境变量切换为 WECHAT_TOOL_BUILD_SESSION_LAST_MESSAGE
- 移除 session_preview_index 实现

src/wechat_decrypt_tool/wechat_decrypt.py

@@ -441,20 +441,20 @@ def decrypt_wechat_databases(db_storage_path: str = None, key: str = None) -> di
             "failed_files": account_failed
         }
 
-        # 构建“会话最后一条消息”索引：把耗时挪到解密阶段，一劳永逸
-        if os.environ.get("WECHAT_TOOL_BUILD_SESSION_PREVIEW", "1") != "0":
+        # 构建“会话最后一条消息”缓存表：把耗时挪到解密阶段，后续会话列表直接查表
+        if os.environ.get("WECHAT_TOOL_BUILD_SESSION_LAST_MESSAGE", "1") != "0":
             try:
-                from .session_preview_index import build_session_preview_index
+                from .session_last_message import build_session_last_message_table
 
-                account_results[account_name]["session_preview_index"] = build_session_preview_index(
+                account_results[account_name]["session_last_message"] = build_session_last_message_table(
                     account_output_dir,
                     rebuild=True,
                     include_hidden=True,
                     include_official=True,
                 )
             except Exception as e:
-                logger.warning(f"构建会话预览索引失败: {account_name}: {e}")
-                account_results[account_name]["session_preview_index"] = {
+                logger.warning(f"构建会话最后一条消息缓存表失败: {account_name}: {e}")
+                account_results[account_name]["session_last_message"] = {
                     "status": "error",
                     "message": str(e),
                 }