Revert coc backfill migration to raw SQL

Author: experimatt

db/migrate/20260526173525_backfill_coc_agreed_at.rb

@@ -1,13 +1,22 @@
 class BackfillCocAgreedAt < ActiveRecord::Migration[7.2]
+  # Raw SQL on purpose: this backfills from code_of_conduct_agreements, a table
+  # slated for removal. Referencing the CodeOfConductAgreement model would break
+  # this migration on replay once that model is deleted.
   def up
-     Participant.where(coc_agreed_at: nil).find_each do |participant|
-       aggreement = CodeOfConductAgreement.where(participant_id: participant.id)
-         .order(created_at: :desc).first
-      participant.update_column(:coc_agreed_at, aggreement.created_at) if aggreement
-    end
+    execute(<<~SQL)
+      UPDATE participants
+      SET coc_agreed_at = coc.last_agreed_at
+      FROM (
+        SELECT participant_id, MAX(created_at) AS last_agreed_at
+        FROM code_of_conduct_agreements
+        GROUP BY participant_id
+      ) AS coc
+      WHERE participants.id = coc.participant_id
+        AND participants.coc_agreed_at IS NULL
+    SQL
   end
 
   def down
-    Participant.update_all(coc_agreed_at: nil)
+    # no-op: backfilled timestamps are indistinguishable from organic agreements
   end
 end