Add sort by relevance(ES score) and position

[Sajgon27]

Taxon position sorting

• Restores position as the default sort on taxon product pages (/taxons/{slug}).
• Added the Position label to the taxon sorting dropdown.

Default search sorting by relevance

• On /search (shop UI and API), results now default to Elasticsearch relevance (_score) instead of newest-first.
• Added SearchProductsSortDataHandler (extends the base above); all other sorters delegate to the parent unchanged.
• Added a Relevance option to the sort dropdown and fixed the sort-label template (it lacked a relevance/position case →
  crashed on select; the default label now reflects the real default sort).
• API: order_by=relevance added to the OpenAPI order_by enum.
• Added ui.relevance and ui.position translations.

Tests

• New phpspec specs for TaxonProductsSortDataHandler and SearchProductsSortDataHandler.
• Existing phpspec + PHPUnit suites pass (no snapshot regressions).