refactor: 项目结构优化与FastAPI生命周期更新

重构项目目录结构，提高代码组织性和可维护性

将schemas目录重命名为domain，更好地表达领域模型概念
将services目录细分为service/chat、service/image等子目录
将api目录重命名为router，更符合FastAPI惯例
创建utils目录存放通用工具函数
更新FastAPI应用程序生命周期管理

替换已弃用的on_event方法为推荐的lifespan事件处理器
添加应用程序关闭时的日志记录
代码质量改进

抽取常量到constants.py，减少硬编码值
添加helpers.py提供通用工具函数
优化配置管理，使用环境变量和默认值
完善文档字符串，提高代码可读性

app/config/config.py

@@ -0,0 +1,61 @@
+"""
+应用程序配置模块
+"""
+from typing import List
+from pydantic_settings import BaseSettings
+
+from app.core.constants import API_VERSION, DEFAULT_MODEL
+
+
+class Settings(BaseSettings):
+    """应用程序配置"""
+    # API相关配置
+    API_KEYS: List[str]
+    ALLOWED_TOKENS: List[str]
+    BASE_URL: str = f"https://generativelanguage.googleapis.com/{API_VERSION}"
+    AUTH_TOKEN: str = ""
+    MAX_FAILURES: int = 3
+    TEST_MODEL: str = DEFAULT_MODEL
+    
+    # 模型相关配置
+    SEARCH_MODELS: List[str] = ["gemini-2.0-flash-exp"]
+    IMAGE_MODELS: List[str] = ["gemini-2.0-flash-exp"]
+    FILTERED_MODELS: List[str] = [
+        "gemini-1.0-pro-vision-latest", 
+        "gemini-pro-vision", 
+        "chat-bison-001", 
+        "text-bison-001", 
+        "embedding-gecko-001"
+    ]
+    TOOLS_CODE_EXECUTION_ENABLED: bool = False
+    SHOW_SEARCH_LINK: bool = True
+    SHOW_THINKING_PROCESS: bool = True
+    
+    # 图像生成相关配置
+    PAID_KEY: str = ""
+    CREATE_IMAGE_MODEL: str = "imagen-3.0-generate-002"
+    UPLOAD_PROVIDER: str = "smms"
+    SMMS_SECRET_TOKEN: str = ""
+    PICGO_API_KEY: str = ""
+    CLOUDFLARE_IMGBED_URL: str = ""
+    CLOUDFLARE_IMGBED_AUTH_CODE: str = ""
+    
+    # 流式输出优化器配置
+    STREAM_MIN_DELAY: float = 0.016
+    STREAM_MAX_DELAY: float = 0.024
+    STREAM_SHORT_TEXT_THRESHOLD: int = 10
+    STREAM_LONG_TEXT_THRESHOLD: int = 50
+    STREAM_CHUNK_SIZE: int = 5
+    
+    def __init__(self, **kwargs):
+        super().__init__(**kwargs)
+        # 设置默认AUTH_TOKEN（如果未提供）
+        if not self.AUTH_TOKEN and self.ALLOWED_TOKENS:
+            self.AUTH_TOKEN = self.ALLOWED_TOKENS[0]
+    
+    class Config:
+        env_file = ".env"
+
+
+# 创建全局配置实例
+settings = Settings()