feat(代码生成器): 重构代码生成模板并增强安全性

重构代码生成模板结构，将模板文件移动到标准目录
添加文件覆盖控制和安全路径检查，防止生成到项目外
优化生成逻辑，支持跳过已存在文件并返回统计信息
完善模板内容，增加分页查询和更多功能实现细节

backend/templates/python/model.py.j2

@@ -1,41 +0,0 @@
-# -*- coding: utf-8 -*-
-
-from datetime import datetime
-from typing import Optional
-from sqlalchemy import String, Integer, Text, DateTime
-from sqlalchemy.orm import Mapped, mapped_column
-
-from app.core.base_model import CreatorMixin
-
-
-class {{ table_name|snake_to_pascal_case }}Model(CreatorMixin):
-    """
-    {{ function_name }}表
-    """
-    
-    __tablename__ = '{{ table_name }}'
-    __table_args__ = {'comment': '{{ function_name }}'}
-
-    {% for column in columns %}
-    {{ column.column_name }}: Mapped[Optional[{{ column.python_type }}]] = mapped_column({{ column.column_type|get_sqlalchemy_type }}, nullable=True, comment='{{ column.column_comment }}')
-    {% endfor %}
-    
-        
-{% if sub_table %}
-
-class {{ sub_class_name }}(Base):
-    """
-    {{ sub_table.function_name }}表
-    """
-    __tablename__ = '{{ sub_table_name }}'
-    __table_args__ = {'comment': '{{ sub_table.function_name }}'}
-    
-    {% for column in sub_table.columns %}
-    {{ column.column_name }}: Mapped[Optional[{{ column.python_type }}]] = mapped_column({{ column.column_type | get_sqlalchemy_type }}, {% if column.column_name == sub_table_fk_name %}ForeignKey('{{ table_name }}.id'), {% endif %}{% if column.pk %}primary_key=True, {% endif %}nullable=True, comment='{{ column.column_comment }}')
-    {% endfor %}
-    
-    {{ table_name }} = relationship('{{ table_name|snake_to_pascal_case }}Model', back_populates='{{ sub_table.table_name }}_list')
-    
-{{ table_name|snake_to_pascal_case }}Model.{{ sub_table.table_name }}_list = relationship('{{ sub_class_name }}', back_populates='{{ table_name }}')
-
-{% endif %}