Allow search to filter by multiple specific sets

[ani-per]

Currently, you can only search inside a single set. You can search across multiple sets if you filter by difficulties, but you can't filter within those multiple sets.

Searching across multiple specific sets is a pretty common desire for writers - e.g. an ACF Fall writer might want to look through clues for a given answerline across all previous ACF Falls.

Currently there doesn't seem to be any organization of sets It'd be particularly helpful if sets were organized database-side like cars' make and models, e.g.:

|-- ACF
|   |-- Fall
|   |   -- 2023
|   |   -- 2024
|   |   -- 2025
|   |-- Winter
|   |   -- 2023
|   |   -- 2024
|   |   -- 2025
|   |-- Regionals
|   |   -- 2023
|   |   -- 2024
|   |   -- 2025
|-- PACE
|   |-- NSC
|   |   -- 2023
|   |   -- 2024
|   |   -- 2025
|-- Chicago Open
|   |-- 2023
|   |-- 2024
|   |-- 2025
|-- Standalone
|   |-- Planetfall
|   |   -- 2023
|   |-- NAFTA
|   |   -- 2025

That way, one could implement a frontend search where users can filter by e.g., ACF, ACF Fall, 2023 ACF Fall, or all Standalone sets.