Migration V30: Move preferred_locale from column to custom_fields JSONB.

This migration removes the dedicated preferred_locale column and stores the value in the existing custom_fields JSONB column instead. This reduces schema complexity by leveraging the flexible JSONB storage for user preferences.

Changes

• Migrates existing preferred_locale values into custom_fields JSONB
• Drops the preferred_locale index
• Drops the preferred_locale column

Requirements

• PostgreSQL database
• PhoenixKit V29 or higher
• The custom_fields JSONB column must exist (added in V18)

Data Migration

Existing preferred_locale values are preserved by copying them into custom_fields:

• user.preferred_locale = "en-GB" becomes user.custom_fields["preferred_locale"] = "en-GB"
• NULL values are not migrated (same behavior: use system default)

Backward Compatibility

• Reading: get_in(user.custom_fields, ["preferred_locale"]) or user.custom_fields["preferred_locale"]
• Writing: Merge into custom_fields map
• NULL handling: Missing key = use system default (same as before)

Rollback

The down migration restores the column and migrates data back from custom_fields.