test: migrate dossier spec examples to seeded records

Replace factory procedure/dossier setup with oaken seeds in sixteen
describe blocks (~55 examples), scope the remaining global
DossierNotification.count assertions, and backdate seeded dossier
traitements by one day so specs freezing time to earlier today still
see them in the past. Full spec runs ~15% faster than the factory
baseline.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: tchak

db/seeds/dossiers.rb

@@ -17,7 +17,9 @@
 
     if state != Dossier.states.fetch(:brouillon)
       dossier.state = state
-      processed_at = DossierWithReferenceDate.assign(dossier)
+      # Deposited dossiers are backdated so that specs freezing time to earlier
+      # today still see the seeded traitements in the past.
+      processed_at = DossierWithReferenceDate.assign(dossier, reference_date: 1.day.ago)
 
       case state
       when Dossier.states.fetch(:en_construction)