feat: Add Files API support with upload, list, get and delete operations

- Implement complete Files API compatible with Gemini API format
- Support resumable file uploads with chunked transfer (tested with 15MB video)
- Create file management service with database tracking
- Add file domain models and API request/response objects
- Implement file routes with proper authentication
- Use fixed API key for Files API requests (due to Google API restrictions)
- Support file state management (PROCESSING, ACTIVE, FAILED)
- Add scheduled task for automatic expired file cleanup
- Integrate seamlessly with existing key management and load balancing

app/domain/file_models.py

@@ -0,0 +1,69 @@
+"""
+Files API 相关的领域模型
+"""
+from typing import Optional, Dict, Any, List
+from datetime import datetime
+from pydantic import BaseModel, Field
+
+
+class FileUploadConfig(BaseModel):
+    """文件上传配置"""
+    mime_type: Optional[str] = Field(None, description="MIME 类型")
+    display_name: Optional[str] = Field(None, description="显示名称，最多40个字符")
+
+
+class CreateFileRequest(BaseModel):
+    """创建文件请求（用于初始化上传）"""
+    file: Optional[Dict[str, Any]] = Field(None, description="文件元数据")
+    
+
+class FileMetadata(BaseModel):
+    """文件元数据响应"""
+    name: str = Field(..., description="文件名称，格式: files/{file_id}")
+    displayName: Optional[str] = Field(None, description="显示名称")
+    mimeType: str = Field(..., description="MIME 类型")
+    sizeBytes: str = Field(..., description="文件大小（字节）")
+    createTime: str = Field(..., description="创建时间 (RFC3339)")
+    updateTime: str = Field(..., description="更新时间 (RFC3339)")
+    expirationTime: str = Field(..., description="过期时间 (RFC3339)")
+    sha256Hash: Optional[str] = Field(None, description="SHA256 哈希值")
+    uri: str = Field(..., description="文件访问 URI")
+    state: str = Field(..., description="文件状态")
+    
+    class Config:
+        json_encoders = {
+            datetime: lambda v: v.isoformat() + "Z"
+        }
+
+
+class ListFilesRequest(BaseModel):
+    """列出文件请求参数"""
+    pageSize: Optional[int] = Field(10, ge=1, le=100, description="每页大小")
+    pageToken: Optional[str] = Field(None, description="分页标记")
+
+
+class ListFilesResponse(BaseModel):
+    """列出文件响应"""
+    files: List[FileMetadata] = Field(default_factory=list, description="文件列表")
+    nextPageToken: Optional[str] = Field(None, description="下一页标记")
+
+
+class UploadInitResponse(BaseModel):
+    """上传初始化响应（内部使用）"""
+    file_metadata: FileMetadata
+    upload_url: str
+
+
+class FileKeyMapping(BaseModel):
+    """文件与 API Key 的映射关系（内部使用）"""
+    file_name: str
+    api_key: str
+    user_token: str
+    created_at: datetime
+    expires_at: datetime
+
+
+class DeleteFileResponse(BaseModel):
+    """删除文件响应"""
+    success: bool = Field(..., description="是否删除成功")
+    message: Optional[str] = Field(None, description="消息")