Stronger security unique index and data migration

Note to self hosters:

If you started self hosting prior to this commit, you may have duplicate securities in your database.

This is usually not a problem, but if you'd like to clean things up, you can run the data migration
by opening a terminal on the machine you're hosting with and running:

```sh
rake data_migration:migrate_duplicate_securities
```

db/migrate/20250522174753_stronger_uniqueness_constraint_on_securities.rb

@@ -0,0 +1,13 @@
+class StrongerUniquenessConstraintOnSecurities < 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