Handle RecordNotFound in ImagesController#solution

[iHiD]

Summary

• Add rescue_from ActiveRecord::RecordNotFound, with: :render_404 to ImagesController
• Solution.for! raises RecordNotFound when given invalid user/track/exercise slugs, which was causing unhandled 500 errors
• Follows the same pattern used in DocsController, LegacyController, etc.

Test plan

• Visit an image solution URL with an invalid user handle, track slug, or exercise slug — should render 404 instead of 500

🤖 Generated with Claude Code