_parse_weights was silently dropping content_class values that don't
belong on their target list with no operator feedback. Changed it to
return (weights, dropped), apply_payload to collect and return all
dropped names, and put_config to include dropped_entries in the
response when non-empty.
get_config was calling planner.apply_payload on every GET request, racing
concurrent reads on module-level globals. Added a _hydrated flag + lock
so DB hydration runs at most once per process lifetime; put_config marks
it done too. Test fixture resets the flag between tests.
