MariaDB JSON columns cause ActiveRecord::ConfigurationError with store_accessor (filters.fields)

Summary

When running Fizzy on MariaDB, requests fail with:

ActiveRecord::ConfigurationError: the column 'fields' has not been configured as a store raised from Filter.from_params. MariaDB exposes JSON as LONGTEXT, so Rails store_accessor doesn't recognize it as a structured column.

Steps to Reproduce

1. Use MariaDB (e.g., 10.6/11.x) with Fizzy
2. Run bin/rails db:prepare
3. Hit /0000001/ or a board page

Temporary Workaround

Adding:

store :fields, coder: JSON

to app/models/filter.rb

and:

store :particulars, coder: JSON

to app/models/event.rb