Add observer transfer ingest and manifest endpoints
Extract shared ingest pipeline (_process_ingest_files) from ingest_upload()
and add POST /ingest/<key>/transfer for receiving pre-processed segments
from other solstone instances. Add GET /ingest/<key>/manifest for day
listing and GET /ingest/<key>/manifest/<day> for per-day transfer manifests.
Add observe.transferred event handler that queues indexer rescan.