Add back schema migration for unique securities

db/migrate/20250522201031_stronger_unique_index_on_security.rb

@@ -0,0 +1,13 @@
+class StrongerUniqueIndexOnSecurity < ActiveRecord::Migration[7.2]
+  def change
+    remove_index :securities, [ :ticker, :exchange_operating_mic ], unique: true
+
+    # Matches our ActiveRecord validation:
+    # - uppercase ticker
+    # - either exchange_operating_mic or empty string (unique index doesn't work with NULL values)
+    add_index :securities,
+              "UPPER(ticker), COALESCE(UPPER(exchange_operating_mic), '')",
+              unique: true,
+              name: "index_securities_on_ticker_and_exchange_operating_mic_unique"
+  end
+end