feat(upload): add resumable chunk configuration and file validation

- Add `WARPBOX_RESUMABLE_CHUNK_MODE` and `WARPBOX_RESUMABLE_CHUNK_PATH` environment variables to configure temporary chunk storage.
- Implement strict file validation for resuming uploads to ensure selected files match the pending session's metadata.
- Add `PLANS.md` to document development stages, roadmap, and API specifications (including batching and resumable flows).

backend/libs/handlers/app.go

@@ -146,6 +146,7 @@ func (a *App) RegisterRoutes(mux *http.ServeMux) {
 	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("POST /api/v1/uploads/resumable/{sessionID}/complete-uploaded", a.CompleteUploadedResumableUpload)
 	mux.HandleFunc("DELETE /api/v1/uploads/resumable/{sessionID}", a.CancelResumableUpload)
 	mux.HandleFunc("GET /emoji/{pack}/{file}", a.EmojiAsset)
 	mux.Handle("GET /static/", a.Static())