test: 建立单元测试基座

- 新增 unittest tests 基座，覆盖 appconfig 与 db 纯逻辑

- 增加 excel/prompts 未实现模块的契约占位测试

- 补强 config.json 对 *_token 与 *_password 的敏感字段拦截

- 更新任务看板、当前状态、API 合约和进度记录

tests/_helpers.py

@@ -0,0 +1,19 @@
+"""Shared unittest helpers."""
+
+import os
+import sys
+import tempfile
+from pathlib import Path
+
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+if str(REPO_ROOT) not in sys.path:
+    sys.path.insert(0, str(REPO_ROOT))
+
+
+class TempDirMixin:
+    def make_temp_dir(self):
+        return tempfile.TemporaryDirectory(prefix="cmshopee_test_", dir=Path(__file__).parent)
+
+    def assert_removed(self, path):
+        self.assertFalse(os.path.exists(path), path)