chore(server): add story background setting

[nina992]

Overview

What I've done

What I haven't done

How I tested

Which point I want you to review particularly

Memo

[netlify]

✅ Deploy Preview for reearth-web canceled.

Name	Link
🔨 Latest commit	eb32f14
🔍 Latest deploy log	https://app.netlify.com/sites/reearth-web/deploys/654479d2e0fd4e00084b66d0

[codecov]

Codecov Report

Merging #774 (ffb9d09) into main (0ba17b4) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is n/a.

❗ Current head ffb9d09 differs from pull request most recent head eb32f14. Consider uploading reports for the commit eb32f14 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #774   +/-   ##
=======================================
  Coverage   26.10%   26.10%           
=======================================
  Files        1621     1621           
  Lines      177201   177201           
  Branches     3905     3905           
=======================================
  Hits        46262    46262           
  Misses     129850   129850           
  Partials     1089     1089           

Flag	Coverage Δ
server	33.37% <ø> (ø)

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