Decide strategy: remove image_url vs keep as legacy (document the decision)

[zigzagdev]

Tasks

• Choose one approach
  • Details
    • Decision: Backend will return thumbnail_url and images[].url only. image_url is deprecated and will not be used by frontend.
    • Compatibility: image_url will be removed after frontend migration (or kept as legacy_image_url for N releases).
• Document migration plan (if necesarry)