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:

```ruby
store :fields, coder: JSON
```
to app/models/filter.rb

and:

```ruby
store :particulars, coder: JSON
```
to app/models/event.rb