chore(server): add bgColor to story

[pyshx]

Overview

This PR adds bgColor in story and the revelant changes has been made to update them through the update mutation on the story.

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[codecov]

Codecov Report

Merging #791 (7df125e) into main (f9e53a5) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #791      +/-   ##
==========================================
- Coverage   25.97%   25.97%   -0.01%     
==========================================
  Files        1630     1630              
  Lines      178051   178056       +5     
  Branches     3913     3913              
==========================================
  Hits        46247    46247              
- Misses     130715   130720       +5     
  Partials     1089     1089              

Flag	Coverage Δ
server	33.37% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
server/pkg/storytelling/story.go	60.34% <0.00%> (-1.06%)	⬇️
server/internal/usecase/interactor/storytelling.go	0.00% <0.00%> (ø)