Add index on url for duplication check

[samgozman]

Closes #86

[codecov]

Codecov Report

Attention: 36 lines in your changes are missing coverage. Please review.

Comparison is base (0eb685b) 38.36% compared to head (a5fca52) 37.78%.

Files	Patch %	Lines
jobs/job.go	0.00%	24 Missing ⚠️
archivist/models/News.go	0.00%	9 Missing and 1 partial ⚠️
journalist/provider.go	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
- Coverage   38.36%   37.78%   -0.59%     
==========================================
  Files          17       17              
  Lines        1843     1874      +31     
==========================================
+ Hits          707      708       +1     
- Misses       1081     1109      +28     
- Partials       55       57       +2     

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