* Move genre-to-genres migration into a dedicated Migration class and
wire it into Library._migrations for items and albums.
* Add batched SQL updates via mutate_many and share the multi-value
delimiter as a constant.
* Cover migration behavior with new tests.
I initially attempted to migrate using our model infrastructure
/ Model.store(), see the comparison below:
Durations migrating my library of ~9000 items and ~2300 albums:
1. Using our Python logic: 11 minutes
2. Using SQL directly: 4 seconds
That's why I've gone ahead with option 2.
