refactor: improve audio processing and upload handling for better performance

Commit notes

- Updated audio processing to complete in parallel with analysis, enhancing upload responsiveness. - Adjusted progress updates to reflect completion accurately. - Enhanced error handling and validation for upload sessions, ensuring idempotency in cancel requests. - Optimized chunk upload concurrency settings to prevent rate limiting and cascading failures.

Files modified: - backend/src/worker.ts - backend/src/routes/uploads.ts - backend/src/storage/sessions.ts - frontend/src/lib/api/upload.ts