db(flags): change unique index name

[wuhan005]

No description provided.

[codecov-commenter]

Codecov Report

Merging #125 (9689565) into master (7f9a8de) will increase coverage by 0.52%.
The diff coverage is 88.00%.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
+ Coverage   70.77%   71.29%   +0.52%     
==========================================
  Files          10       10              
  Lines         414      432      +18     
==========================================
+ Hits          293      308      +15     
- Misses        100      102       +2     
- Partials       21       22       +1     

Flag	Coverage Δ
unittests	71.29% <88.00%> (+0.52%)	⬆️

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

Impacted Files	Coverage Δ
internal/db/flag.go	82.97% <ø> (ø)
internal/db/actions.go	78.12% <88.00%> (+2.03%)	⬆️

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