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/service/client/api_client.py

@@ -0,0 +1,61 @@
+# app/services/chat/api_client.py
+
+from typing import Dict, Any, AsyncGenerator
+import httpx
+from abc import ABC, abstractmethod
+
+from app.core.constants import DEFAULT_TIMEOUT
+
+
+class ApiClient(ABC):
+    """API客户端基类"""
+
+    @abstractmethod
+    async def generate_content(self, payload: Dict[str, Any], model: str, api_key: str) -> Dict[str, Any]:
+        pass
+
+    @abstractmethod
+    async def stream_generate_content(self, payload: Dict[str, Any], model: str, api_key: str) -> AsyncGenerator[str, None]:
+        pass
+
+
+class GeminiApiClient(ApiClient):
+    """Gemini API客户端"""
+
+    def __init__(self, base_url: str, timeout: int = DEFAULT_TIMEOUT):
+        self.base_url = base_url
+        self.timeout = timeout
+
+    def _get_real_model(self, model: str) -> str:
+        if model.endswith("-search"):
+            model = model[:-7]
+        if model.endswith("-image"):
+            model = model[:-6]
+
+        return model
+
+    async def generate_content(self, payload: Dict[str, Any], model: str, api_key: str) -> Dict[str, Any]:
+        timeout = httpx.Timeout(self.timeout, read=self.timeout)
+        model = self._get_real_model(model)
+
+        async with httpx.AsyncClient(timeout=timeout) as client:
+            url = f"{self.base_url}/models/{model}:generateContent?key={api_key}"
+            response = await client.post(url, json=payload)
+            if response.status_code != 200:
+                error_content = response.text
+                raise Exception(f"API call failed with status code {response.status_code}, {error_content}")
+            return response.json()
+
+    async def stream_generate_content(self, payload: Dict[str, Any], model: str, api_key: str) -> AsyncGenerator[str, None]:
+        timeout = httpx.Timeout(self.timeout, read=self.timeout)
+        model = self._get_real_model(model)
+        
+        async with httpx.AsyncClient(timeout=timeout) as client:
+            url = f"{self.base_url}/models/{model}:streamGenerateContent?alt=sse&key={api_key}"
+            async with client.stream(method="POST", url=url, json=payload) as response:
+                if response.status_code != 200:
+                    error_content = await response.aread()
+                    error_msg = error_content.decode("utf-8")
+                    raise Exception(f"API call failed with status code {response.status_code}, {error_msg}")
+                async for line in response.aiter_lines():
+                    yield line