Upcoming Queue: fix the last few todos (groupwise-max queries)

[mna]

For #23916 , follow-up of PR comments on #26067 and others.

Checklist for submitter

• Input data is properly validated, SELECT * is avoided, SQL injection is prevented (using placeholders for values in statements)
• Added/updated automated tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.85%. Comparing base (194db62) to head (3cba03a).
Report is 1 commits behind head on feat-upcoming-activites-queue.

Additional details and impacted files

@@                        Coverage Diff                        @@
##           feat-upcoming-activites-queue   #26107      +/-   ##
=================================================================
- Coverage                          63.86%   63.85%   -0.01%     
=================================================================
  Files                               1632     1632              
  Lines                             157875   157852      -23     
  Branches                            4120     4093      -27     
=================================================================
- Hits                              100820   100792      -28     
- Misses                             49153    49155       +2     
- Partials                            7902     7905       +3     

Flag	Coverage Δ
backend	64.70% <100.00%> (-0.01%)	⬇️

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