feat(auth): add wechat miniapp customer phone login

Implement Wechat Mini Program phone number login flow for end customers:
- add PhoneLoginIn Pydantic request schema
- create wechat.py module for Wechat API interactions and phone code exchange
- add customer JWT utilities and require_customer authentication dependency
- add new public API endpoints: /api/public/auth/phone-login and /api/public/auth/me
- add required environment config variables and update example .env
- add comprehensive test cases for the new auth flow and endpoints

.env.example

@@ -7,3 +7,6 @@ OSS_ACCESS_KEY_ID="your-oss-access-key-id"
 OSS_ACCESS_KEY_SECRET="your-oss-access-key-secret"
 OSS_ENDPOINT="your-oss-endpoint"
 OSS_BUCKET_NAME="your-oss-bucket-name"
+WECHAT_MINIAPP_APPID="your-miniapp-appid"
+WECHAT_MINIAPP_SECRET="your-miniapp-secret"
+CUSTOMER_JWT_EXPIRES_HOURS=720