Locks the one-time backfill that derives verification_status for pre-column
library_history rows from acoustid_result: pass->verified, skip->unverified,
fail->force_imported; never overwrites an existing status (NULL-only); leaves
no-acoustid rows NULL; idempotent across repeated inits. Exercises the real
_initialize_database path (clears the per-process init guard to re-trigger it).
