Tech: corrige le backfill des notifications pour les cas où un dossier doit avoir la même notif pour plusieurs instructeurs

[colinux]

c'est le .find_or_create_by qui se charge dans le process de pas créer de doublon

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.94%. Comparing base (e93f84b) to head (6833f78).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11675      +/-   ##
==========================================
+ Coverage   59.11%   64.94%   +5.83%     
==========================================
  Files        1255     1255              
  Lines       27620    27620              
  Branches     5683     5683              
==========================================
+ Hits        16327    17939    +1612     
+ Misses      11293     9681    -1612     

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