Add contest duration & complete ranking mode implementation

db/migrate/20251118074900_add_duration_to_contest.rb

@@ -0,0 +1,5 @@
+class AddDurationToContest < ActiveRecord::Migration[8.0]
+  def change
+    add_column :contests, :duration, :string
+  end
+end

db/migrate/20251118074914_add_duration_seconds_to_contest.rb

@@ -0,0 +1,5 @@
+class AddDurationSecondsToContest < ActiveRecord::Migration[8.0]
+  def change
+    add_column :contests, :duration_seconds, :integer
+  end
+end