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/domain/gemini_models.py

@@ -0,0 +1,40 @@
+from typing import List, Optional, Dict, Any, Literal
+from pydantic import BaseModel
+
+
+class SafetySetting(BaseModel):
+    category: Optional[Literal["HARM_CATEGORY_HATE_SPEECH", "HARM_CATEGORY_DANGEROUS_CONTENT", "HARM_CATEGORY_HARASSMENT", "HARM_CATEGORY_SEXUALLY_EXPLICIT", "HARM_CATEGORY_CIVIC_INTEGRITY"]] = None
+    threshold: Optional[Literal["HARM_BLOCK_THRESHOLD_UNSPECIFIED", "BLOCK_LOW_AND_ABOVE", "BLOCK_MEDIUM_AND_ABOVE", "BLOCK_ONLY_HIGH", "BLOCK_NONE", "OFF"]] = None
+
+
+class GenerationConfig(BaseModel):
+    stopSequences: Optional[List[str]] = None
+    responseMimeType: Optional[str] = None
+    responseSchema: Optional[Dict[str, Any]] = None
+    candidateCount: Optional[int] = 1
+    maxOutputTokens: Optional[int] = None
+    temperature: Optional[float] = None
+    topP: Optional[float] = None
+    topK: Optional[int] = None
+    presencePenalty: Optional[float] = None
+    frequencyPenalty: Optional[float] = None
+    responseLogprobs: Optional[bool] = None
+    logprobs: Optional[int] = None
+
+
+class SystemInstruction(BaseModel):
+    role: str = "system"
+    parts: List[Dict[str, Any]]
+
+
+class GeminiContent(BaseModel):
+    role: str
+    parts: List[Dict[str, Any]]
+
+
+class GeminiRequest(BaseModel):
+    contents: List[GeminiContent] = []
+    tools: Optional[List[Dict[str, Any]]] = []
+    safetySettings: Optional[List[SafetySetting]] = None
+    generationConfig: Optional[GenerationConfig] = None
+    systemInstruction: Optional[SystemInstruction] = None