feat(uploads): add native resumable upload support

Implement a native chunked resumable upload API and frontend integration
to support reliable large file uploads.

Changes include:
- Added a 3-step resumable upload API flow (create session, upload chunks, complete session).
- Introduced configuration options for chunk size, retention hours, and toggling the feature.
- Updated the frontend to utilize resumable uploads with progress tracking.
- Configured temporary chunk storage under `data/tmp/uploads` with automatic cleanup.
- Documented the API flow and configuration in the README.

backend/libs/handlers/app.go

@@ -141,6 +141,12 @@ func (a *App) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("GET /api/v1/schemas/upload-request.json", a.UploadRequestSchema)
 	mux.HandleFunc("GET /api/v1/schemas/upload-response.json", a.UploadResponseSchema)
 	mux.HandleFunc("POST /api/v1/upload", a.Upload)
+	mux.HandleFunc("POST /api/v1/uploads/resumable", a.CreateResumableUpload)
+	mux.HandleFunc("GET /api/v1/uploads/resumable/{sessionID}", a.ResumableUploadStatus)
+	mux.HandleFunc("POST /api/v1/uploads/resumable/{sessionID}/files", a.AddResumableFiles)
+	mux.HandleFunc("PUT /api/v1/uploads/resumable/{sessionID}/files/{fileID}/chunks/{index}", a.PutResumableChunk)
+	mux.HandleFunc("POST /api/v1/uploads/resumable/{sessionID}/complete", a.CompleteResumableUpload)
+	mux.HandleFunc("DELETE /api/v1/uploads/resumable/{sessionID}", a.CancelResumableUpload)
 	mux.HandleFunc("GET /emoji/{pack}/{file}", a.EmojiAsset)
 	mux.Handle("GET /static/", a.Static())
 }