feat: AI改写功能集成角色数据 + UI优化

- 新增story_characters表和seed_characters.sql种子数据(27个角色)
- AI改写/续写功能注入角色信息(性别/年龄/外貌/性格)
- 首页UI下移避让微信退出按钮
- 个人中心页面布局重构

server/app/models/story.py

@@ -26,6 +26,28 @@ class Story(Base):
     updated_at = Column(TIMESTAMP, server_default=func.now(), onupdate=func.now())
     
     nodes = relationship("StoryNode", back_populates="story", cascade="all, delete-orphan")
+    characters = relationship("StoryCharacter", back_populates="story", cascade="all, delete-orphan")
+
+
+class StoryCharacter(Base):
+    """故事角色表"""
+    __tablename__ = "story_characters"
+    
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    story_id = Column(Integer, ForeignKey("stories.id", ondelete="CASCADE"), nullable=False)
+    name = Column(String(50), nullable=False)
+    role_type = Column(String(20), default="supporting")  # protagonist/antagonist/supporting
+    gender = Column(String(10), default="")
+    age_range = Column(String(20), default="")
+    appearance = Column(Text)  # 外貌描述
+    personality = Column(Text)  # 性格描述
+    background = Column(Text)  # 背景故事
+    avatar_prompt = Column(Text)  # AI绘图提示词
+    avatar_url = Column(String(500), default="")
+    created_at = Column(TIMESTAMP, server_default=func.now())
+    updated_at = Column(TIMESTAMP, server_default=func.now(), onupdate=func.now())
+    
+    story = relationship("Story", back_populates="characters")
 
 
 class StoryNode(Base):