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

.env.example

@@ -79,4 +79,12 @@ URL_NORMALIZATION_ENABLED=false
 # tts配置
 TTS_MODEL=gemini-2.5-flash-preview-tts
 TTS_VOICE_NAME=Zephyr
-TTS_SPEED=normal
+TTS_SPEED=normal
+#########################Files API 相关配置########################
+# 是否启用文件过期自动清理
+FILES_CLEANUP_ENABLED=true
+# 文件过期清理间隔（小时）
+FILES_CLEANUP_INTERVAL_HOURS=1
+# 是否启用用户文件隔离（每个用户只能看到自己上传的文件）
+FILES_USER_ISOLATION_ENABLED=true
+##########################################################################