feat(home): optimize general_stats route

[jacquesfize]

closes #3309

Based on propositions made by @dba-sig-sfepm

Includes :

• update of the general_stats route
• new benchmark test
• new index on the observers column of the gn_synthese.synthese column

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.93%. Comparing base (6f6b91a) to head (5d27a81).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3313   +/-   ##
========================================
  Coverage    83.93%   83.93%           
========================================
  Files          122      122           
  Lines         9715     9716    +1     
========================================
+ Hits          8154     8155    +1     
  Misses        1561     1561           

Flag	Coverage Δ
pytest	83.93% <100.00%> (+<0.01%)	⬆️

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