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/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.2].define(version: 2025_05_21_112347) do
+ActiveRecord::Schema[7.2].define(version: 2025_05_22_174753) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pgcrypto"
   enable_extension "plpgsql"
@@ -517,9 +517,9 @@ ActiveRecord::Schema[7.2].define(version: 2025_05_21_112347) do
     t.datetime "failed_fetch_at"
     t.integer "failed_fetch_count", default: 0, null: false
     t.datetime "last_health_check_at"
+    t.index "upper((ticker)::text), COALESCE(upper((exchange_operating_mic)::text), ''::text)", name: "index_securities_on_ticker_and_exchange_operating_mic_unique", unique: true
     t.index ["country_code"], name: "index_securities_on_country_code"
     t.index ["exchange_operating_mic"], name: "index_securities_on_exchange_operating_mic"
-    t.index ["ticker", "exchange_operating_mic"], name: "index_securities_on_ticker_and_exchange_operating_mic", unique: true
   end
 
   create_table "security_prices", id: :uuid, default: -> { "gen_random_uuid()" }, force: :cascade do |t|