Remove trailing slash from staging STORE_UPLOAD_URL.

[Roadmaster]

The trailing slash results in:

simplejson.errors.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

because the upload path contains a double slash, in the server logs we see:

POST //unscanned-upload/ HTTP/1.1...

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[codecov-commenter]

Codecov Report

Merging #3876 (c64c41b) into main (b962783) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3876   +/-   ##
=======================================
  Coverage   88.59%   88.59%           
=======================================
  Files          79       79           
  Lines        4842     4842           
=======================================
  Hits         4290     4290           
  Misses        552      552           

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