refactor: 第二轮架构缺陷修复 (1/2/3/4/9/10)

1. 内存泄漏修复：RealtimeChatManager 添加会话自动清理机制
   - 每10次操作检查超时会话（1小时无活动自动清理）
   - 最大活跃会话数限制500，超限清理最旧会话

2. 数据库索引补全：
   - Conversation: session_id, work_order_id 添加索引
   - WorkOrder: status 添加索引
   - ChatSession: user_id 添加索引
   - KnowledgeEntry: category, is_active, is_verified 添加索引

3. ServiceManager 线程安全：
   - 添加 threading.Lock 双重检查锁
   - 防止多线程并发初始化同一服务

4. API 响应格式统一：
   - 新增 api_response() 标准响应函数
   - 统一格式: {success, message, data} / {success, error}

9. asyncio 误用修复：
   - knowledge.py 文件上传改用安全的 asyncio 调用方式
   - 兼容已有事件循环和无事件循环两种场景

10. 请求限流：
    - 新增 rate_limit 装饰器（按 IP 限流）
    - chat/message 限制 20次/分钟
    - workorder/ai-suggestion 限制 5次/分钟

src/core/models.py

@@ -49,7 +49,7 @@ class WorkOrder(Base):
     description = Column(Text, nullable=False)
     category = Column(String(100), nullable=False)
     priority = Column(String(20), nullable=False)
-    status = Column(String(20), nullable=False)
+    status = Column(String(20), nullable=False, index=True)
     created_at = Column(DateTime, default=datetime.now)
     updated_at = Column(DateTime, default=datetime.now, onupdate=datetime.now)
     resolution = Column(Text)
@@ -97,8 +97,8 @@ class ChatSession(Base):
 
     id = Column(Integer, primary_key=True)
     tenant_id = Column(String(50), nullable=False, default=DEFAULT_TENANT, index=True)
-    session_id = Column(String(100), unique=True, nullable=False)  # 唯一会话标识
-    user_id = Column(String(100), nullable=True)  # 用户标识
+    session_id = Column(String(100), unique=True, nullable=False)
+    user_id = Column(String(100), nullable=True, index=True)
     work_order_id = Column(Integer, ForeignKey("work_orders.id"), nullable=True)
     title = Column(String(200), nullable=True)  # 会话标题（取首条消息摘要）
     status = Column(String(20), default="active")  # active, ended
@@ -135,8 +135,8 @@ class Conversation(Base):
 
     id = Column(Integer, primary_key=True)
     tenant_id = Column(String(50), nullable=False, default=DEFAULT_TENANT, index=True)
-    session_id = Column(String(100), ForeignKey("chat_sessions.session_id"), nullable=True)  # 关联会话
-    work_order_id = Column(Integer, ForeignKey("work_orders.id"))
+    session_id = Column(String(100), ForeignKey("chat_sessions.session_id"), nullable=True, index=True)
+    work_order_id = Column(Integer, ForeignKey("work_orders.id"), index=True)
     user_message = Column(Text, nullable=False)
     assistant_response = Column(Text, nullable=False)
     timestamp = Column(DateTime, default=datetime.now)
@@ -162,13 +162,13 @@ class KnowledgeEntry(Base):
     tenant_id = Column(String(50), nullable=False, default=DEFAULT_TENANT, index=True)
     question = Column(Text, nullable=False)
     answer = Column(Text, nullable=False)
-    category = Column(String(100), nullable=False)
+    category = Column(String(100), nullable=False, index=True)
     confidence_score = Column(Float, default=0.0)
     usage_count = Column(Integer, default=0)
     created_at = Column(DateTime, default=datetime.now)
     updated_at = Column(DateTime, default=datetime.now, onupdate=datetime.now)
-    is_active = Column(Boolean, default=True)
-    is_verified = Column(Boolean, default=False)  # 是否已验证
+    is_active = Column(Boolean, default=True, index=True)
+    is_verified = Column(Boolean, default=False, index=True)
     verified_by = Column(String(100))  # 验证人
     verified_at = Column(DateTime)  # 验证时间
     vector_embedding = Column(Text)  # 向量嵌入的JSON字符串