feat: ip_addersses.ip_address をユニークに（#229） (#237)

#229

Co-authored-by: miteruzo <miteruzo@naver.com>
Reviewed-on: #237

backend/db/migrate/20260126124100_add_unique_index_to_ip_addresses.rb

@@ -0,0 +1,3 @@
+class AddUniqueIndexToIpAddresses < ActiveRecord::Migration[8.0]
+  add_index :ip_addresses, :ip_address, unique: true, name: 'index_ip_addresses_on_ip_address'
+end