[1.4-maint] json: support BORG_JSON_INDENT env var for JSON output formatting

[charmikadi]

Description

Backport of #9855 to the 1.4-maint branch

Adds support for BORG_JSON_INDENT env var, allowing users to control JSON output indentation (including compact single-line output) for easier parsing by log collectors.

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.17%. Comparing base (e3f7eb7) to head (c3accf2).
⚠️ Report is 3 commits behind head on 1.4-maint.

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint    #9861      +/-   ##
=============================================
+ Coverage      82.04%   82.17%   +0.13%     
=============================================
  Files             38       38              
  Lines          11383    11396      +13     
  Branches        1794     1797       +3     
=============================================
+ Hits            9339     9365      +26     
+ Misses          1461     1449      -12     
+ Partials         583      582       -1     

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[ThomasWaldmann]

Thanks @charmikadi!