Add default_search to user_settings

Add migration to add a nullable VARCHAR(32) column with a CHECK
constraint restricting values to book, author, series, translator.
Update SQLx query artifacts, handlers, and serializers to read and
write the new field.

migrations/20240101000006_add_default_search_setting.sql

@@ -0,0 +1,4 @@
+-- Add default search setting: NULL = not selected, or one of: book, author, series, translator
+ALTER TABLE user_settings
+ADD COLUMN default_search VARCHAR(32) NULL
+CHECK (default_search IS NULL OR default_search IN ('book', 'author', 'series', 'translator'));