artists.genres / albums.genres stored EITHER a JSON array (new writes) OR a
legacy comma-separated string (old writes), forcing every reader to
try-JSON-then-split. Add a marker-gated one-time migration
(_normalize_genres_to_json) that rewrites legacy rows to JSON in place,
mirroring the readers' exact parse (JSON list, else comma-split/strip/
drop-empties) so genre VALUES are unchanged — only the storage format.
Per-row diffed (already-canonical rows untouched, no churn) and non-fatal on
error, consistent with the other migrations. Readers still tolerate both
formats, so this breaks nothing; it just removes the dual-format debt.
Tests: tests/test_db_genres_json_normalization.py — CSV->JSON, JSON-unchanged,
whitespace/empties dropped, albums table, legacy-reader-equivalence,
idempotent re-run, marker set on fresh init.
c5b02c0026