[SandObserver]

Summary

• Cache the Unsplash photo URL for 24 hours, keyed on API key and collection, and share one in-flight request.
• Return a fixed No image returned error and log the upstream status instead of returning Unsplash's error text.

Testing
All green. Added 4 tests for the cache, request sharing and error mapping.

Changelog

Changed

• The Unsplash wallpaper changes once a day instead of on every load.

Security

• The wallpaper endpoint no longer returns Unsplash's error text.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.