RingMarkerItem expects logo_url but backend MapMarkerItem does not include it

## Bug Frontend RingMarkerItem type includes logo_url: string | null but backend MapMarkerItem schema has no logo_url field. ## Impact Ring markers on the discover map never show ring logos. The field is always undefined. ## Fix Add logo_url to backend MapMarkerItem schema (similar to #4083 which adds cover_thumbnail_url for events). Note: Related to #4212 (logo_url missing in org profile group serialization). ## Found by API contract audit (2026-03-19)