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

app/schemas.py

@@ -12,6 +12,10 @@ class LoginIn(BaseModel):
     password: str = Field(min_length=6)
 
 
+class PhoneLoginIn(BaseModel):
+    code: str = Field(min_length=1, max_length=256)
+
+
 class ProductImageIn(BaseModel):
     url: str = ""
     alt: str | None = None