feat: Step 1 - 添加 Bot 数据模型 (v0.9.0)

- 创建 Bot 模型：id/name/display_name/avatar/description
- 添加 owner_id/agent_id/token_hash/is_system 字段
- 添加 status/capabilities/config 字段
- 修改 Session 模型：添加 bot_id 关联
- 修改 Message 模型：添加 sender_name 和 bot_id
- 创建数据库迁移脚本

技术方案实现：
- Bot 一对一绑定 Agent
- owner_id 区分所有权
- is_system 支持系统级 Bot
- capabilities 存储能力标签

app/models/__init__.py

@@ -129,12 +129,57 @@ class Agent(db.Model):
         }
 
 
+class Bot(db.Model):
+    """机器人模型 - Agent 的用户侧展示配置"""
+    __tablename__ = 'bots'
+    
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=generate_uuid)
+    name: Mapped[str] = mapped_column(String(80), unique=True, nullable=False)
+    display_name: Mapped[Optional[str]] = mapped_column(String(80), nullable=True)
+    avatar: Mapped[Optional[str]] = mapped_column(String(256), nullable=True)
+    description: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
+    owner_id: Mapped[str] = mapped_column(String(36), ForeignKey('users.id'), nullable=False)
+    agent_id: Mapped[Optional[str]] = mapped_column(String(36), ForeignKey('agents.id'), nullable=True)
+    token_hash: Mapped[Optional[str]] = mapped_column(String(256), nullable=True)
+    is_system: Mapped[bool] = mapped_column(Boolean, default=False)
+    status: Mapped[str] = mapped_column(String(20), default='offline')
+    capabilities: Mapped[Optional[str]] = mapped_column(JSON, nullable=True)
+    config: Mapped[Optional[str]] = mapped_column(JSON, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow)
+    last_active_at: Mapped[Optional[datetime]] = mapped_column(DateTime, nullable=True)
+    
+    # 关联
+    owner = relationship('User', foreign_keys=[owner_id])
+    agent = relationship('Agent', foreign_keys=[agent_id])
+    
+    def __repr__(self):
+        return f'<Bot {self.name}>'
+    
+    def to_dict(self):
+        return {
+            'id': self.id,
+            'name': self.name,
+            'display_name': self.display_name,
+            'avatar': self.avatar,
+            'description': self.description,
+            'owner_id': self.owner_id,
+            'agent_id': self.agent_id,
+            'is_system': self.is_system,
+            'status': self.status,
+            'capabilities': self.capabilities,
+            'config': self.config,
+            'created_at': self.created_at.isoformat() if self.created_at else None,
+            'last_active_at': self.last_active_at.isoformat() if self.last_active_at else None,
+        }
+
+
 class Session(db.Model):
     """会话模型"""
     __tablename__ = 'sessions'
     
     id: Mapped[str] = mapped_column(String(36), primary_key=True, default=generate_uuid)
     user_id: Mapped[str] = mapped_column(String(36), ForeignKey('users.id'), nullable=False)
+    bot_id: Mapped[Optional[str]] = mapped_column(String(36), ForeignKey('bots.id'), nullable=True)
     primary_agent_id: Mapped[Optional[str]] = mapped_column(String(36), ForeignKey('agents.id'), nullable=True)
     participating_agent_ids: Mapped[Optional[str]] = mapped_column(JSON, nullable=True)
     user_socket_id: Mapped[Optional[str]] = mapped_column(String(100), nullable=True)
@@ -149,6 +194,7 @@ class Session(db.Model):
     
     # 关联
     user = relationship('User', back_populates='sessions')
+    bot = relationship('Bot', foreign_keys=[bot_id])
     agent = relationship('Agent', back_populates='sessions')
     messages = relationship('Message', back_populates='session', cascade='all, delete-orphan')
     
@@ -159,6 +205,7 @@ class Session(db.Model):
         return {
             'id': self.id,
             'user_id': self.user_id,
+            'bot_id': self.bot_id,
             'primary_agent_id': self.primary_agent_id,
             'participating_agent_ids': self.participating_agent_ids,
             'user_socket_id': self.user_socket_id,
@@ -181,6 +228,8 @@ class Message(db.Model):
     session_id: Mapped[str] = mapped_column(String(36), ForeignKey('sessions.id'), nullable=False)
     sender_type: Mapped[str] = mapped_column(String(20), nullable=False)
     sender_id: Mapped[str] = mapped_column(String(36), nullable=False)
+    sender_name: Mapped[Optional[str]] = mapped_column(String(80), nullable=True)
+    bot_id: Mapped[Optional[str]] = mapped_column(String(36), ForeignKey('bots.id'), nullable=True)
     message_type: Mapped[str] = mapped_column(String(20), default='text')
     content: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
     content_type: Mapped[str] = mapped_column(String(20), default='markdown')
@@ -193,6 +242,7 @@ class Message(db.Model):
     
     # 关联
     session = relationship('Session', back_populates='messages')
+    bot = relationship('Bot', foreign_keys=[bot_id])
     
     def __repr__(self):
         return f'<Message {self.id}>'
@@ -203,6 +253,8 @@ class Message(db.Model):
             'session_id': self.session_id,
             'sender_type': self.sender_type,
             'sender_id': self.sender_id,
+            'sender_name': self.sender_name,
+            'bot_id': self.bot_id,
             'message_type': self.message_type,
             'content': self.content,
             'content_type': self.content_type,