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
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