Add log system with three log types (agent/system/chat)

Implemented a complete log system for tracking:
- Agent logs:智能体调用
- System logs: 系统运行
- Chat logs: 问答对话

Backend:
- Log model with type, level, user_id, message, source, duration_ms
- LogService with methods for logging and querying
- API endpoints: GET /api/logs, GET /api/logs/stats, GET /api/logs/recent

Frontend:
- LogView.vue with filters, stats, pagination, auto-refresh
- log.ts API client with TypeScript interfaces
- Added "运行日志" nav item to sidebar

backend/app/models/log.py

@@ -0,0 +1,33 @@
+from sqlalchemy import Column, String, Text, DateTime, Index, Enum as SQLEnum
+from app.models.base import BaseModel
+import enum
+
+
+class LogLevel(str, enum.Enum):
+    DEBUG = "debug"
+    INFO = "info"
+    WARNING = "warning"
+    ERROR = "error"
+
+
+class LogType(str, enum.Enum):
+    AGENT = "agent"      # 智能体调用
+    SYSTEM = "system"    # 系统运行
+    CHAT = "chat"       # 问答对话
+
+
+class Log(BaseModel):
+    __tablename__ = "logs"
+
+    level = Column(String(20), default=LogLevel.INFO.value, index=True)  # debug/info/warning/error
+    type = Column(String(20), default=LogType.SYSTEM.value, index=True)   # agent/system/chat
+    user_id = Column(String(36), nullable=True, index=True)  # 关联用户
+    message = Column(Text, nullable=False)  # 日志内容
+    details = Column(Text, nullable=True)  # 详细信息(JSON)
+    source = Column(String(100), nullable=True)  # 来源模块
+    duration_ms = Column(String(20), nullable=True)  # 执行耗时
+
+    __table_args__ = (
+        Index('idx_logs_type_level', 'type', 'level'),
+        Index('idx_logs_created_at', 'created_at'),
+    )