Fix flow background update

[larsl-net]

This change fixes #482 so that the flow background can be changed.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.49%. Comparing base (1ec31e9) to head (b338c17).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/provider/resource_flow.go	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #566      +/-   ##
==========================================
- Coverage   83.53%   83.49%   -0.04%     
==========================================
  Files          96       96              
  Lines        9290     9296       +6     
==========================================
+ Hits         7760     7762       +2     
- Misses       1076     1078       +2     
- Partials      454      456       +2     

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