Implement missing & remaining pieces propagation + cleaner forms

db/migrate/20251110100516_add_missing_pieces_to_completion.rb

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+class AddMissingPiecesToCompletion < ActiveRecord::Migration[8.0]
+  def change
+    add_column :completions, :missing_pieces, :integer
+  end
+end

db/migrate/20251110103247_add_completed_to_completion.rb

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+class AddCompletedToCompletion < ActiveRecord::Migration[8.0]
+  def change
+    add_column :completions, :completed, :boolean
+  end
+end

db/migrate/20251110110151_add_submitted_to_offline.rb

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+class AddSubmittedToOffline < ActiveRecord::Migration[8.0]
+  def change
+    add_column :offlines, :submitted, :boolean
+  end
+end