fix: 将平台业务ID统一改为String并自动建表

- 抖音、B站、快手、微博的帖子/视频/评论ID从BigInteger改为String，
  避免PostgreSQL下字符串写入BIGINT报错及未来ID溢出风险
- B站dynamic_id改为String，修复API返回id_str被强转int导致的精度丢失
- 知乎提取器对content_id/question_id显式str()转换
- main.py启动数据库保存模式时自动建表，无需手动--init_db
- 同步更新相关老化测试

tests/test_store_factory.py

@@ -33,6 +33,7 @@ from store.xhs._store_impl import (
     XhsMongoStoreImplement,
     XhsExcelStoreImplement
 )
+from store.excel_store_base import ExcelStoreBase
 
 
 class TestXhsStoreFactory:
@@ -73,7 +74,8 @@ class TestXhsStoreFactory:
         """Test creating Excel store"""
         # ContextVar cannot be mocked, so we test with actual value
         store = XhsStoreFactory.create_store()
-        assert isinstance(store, XhsExcelStoreImplement)
+        # XhsExcelStoreImplement 返回 ExcelStoreBase 单例，实际类型为 ExcelStoreBase
+        assert isinstance(store, ExcelStoreBase)
 
     @patch('config.SAVE_DATA_OPTION', 'jsonl')
     def test_create_jsonl_store(self):