[SandObserver]

Summary

• Set client_body_buffer_size 3m on the /api/ location, matching client_max_body_size.
• A config save over nginx's 16 KB default was written to /var/lib/nginx/tmp/client_body/ and logged a warning on every save. The wallpaper upload location keeps the default.
• Add a test that the /api/ buffer is at least the API's own body limit.

Testing
All green, e2e included. Added 1 nginx config test.

Changelog

Fixed

• Stop logging an nginx warning on every settings save.

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.