Registering thousands of extracted frames one-by-one via POST /frames
doesn't scale, so add `spelunkai-labeling-backend ingest-session`: it
writes Frame rows directly against the database (no server needs to
be running) for all frame_*.png files under a directory already
placed under FRAMES_ROOT. Idempotent per (session_name, frame_index),
so re-running after copying more frames only inserts the new ones.
Restructured the CLI into subcommands (serve / ingest-session) while
keeping `spelunkai-labeling-backend` with no arguments working exactly
as before (defaults to serve), verified against a live run. 22/22
backend tests pass.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
