refactor: remove legacy user registration from auth_service

- Remove unused register_user() method and helper methods
- Remove legacy UserRegister schema (customer registration uses CustomerService)
- Remove wrapper methods that just delegated to auth_manager
- Simplify auth_service to focus on login and vendor access control
- Clean up tests to match simplified service

The only registration path is now /api/v1/shop/auth/register for customers,
which uses CustomerService.register_customer().

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

models/schema/auth.py

@@ -5,30 +5,6 @@ from datetime import datetime
 from pydantic import BaseModel, ConfigDict, EmailStr, Field, field_validator
 
 
-class UserRegister(BaseModel):
-    email: EmailStr = Field(..., description="Valid email address")
-    username: str = Field(..., description="Username")
-    password: str = Field(..., description="Password")
-
-    # Keep security validation in Pydantic for auth
-
-    @field_validator("username")
-    @classmethod
-    def validate_username(cls, v):
-        if not re.match(r"^[a-zA-Z0-9_]+$", v):
-            raise ValueError(
-                "Username must contain only letters, numbers, or underscores"
-            )
-        return v.lower().strip()
-
-    @field_validator("password")
-    @classmethod
-    def validate_password(cls, v):
-        if len(v) < 6:
-            raise ValueError("Password must be at least 6 characters long")
-        return v
-
-
 class UserLogin(BaseModel):
     email_or_username: str = Field(..., description="Username or email address")
     password: str = Field(..., description="Password")