feat(records): async Hetzner object storage upload via Celery

Summary

• Moves file upload to Hetzner S3-compatible object storage into a background Celery task (upload_record_to_storage)
• Introduces UploadJob model to track async upload state (status, progress, error, linked record)
• Adds GET /upload/{upload_id}/status endpoint for clients to poll job progress
• Adds UPLOADS_ASYNC_ENABLED feature flag (default: true) to toggle sync/async behaviour
• Adds Alembic migration c1d2e3f4a5b6 to create the uploadjob table

Test plan

• Upload audio file in chunks via POST /records/upload/chunk
• Finalize upload via POST /records/upload/finalize — confirm 202 Accepted with upload_id
• Poll GET /upload/{upload_id}/status — confirm status progresses (queued → processing → completed/failed)
• Confirm record is created in DB with record_id populated in upload job on success
• Set UPLOADS_ASYNC_ENABLED=false — confirm upload runs synchronously (no Celery task)
• Run alembic upgrade head — confirm uploadjob table created cleanly

Closes #63