FIX: Remove duplicates before adding index (#315)

Turns out there are one or two duplicates on some sites, so we have to delete them prior to adding the index.

Author: nattsw

db/migrate/20250630040849_add_index_post_locale_post_id.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+class ClearDupesAddIndexToPostLocale < ActiveRecord::Migration[7.2]
+  disable_ddl_transaction!
+
+  def up
+    execute <<~SQL
+      DELETE FROM discourse_translator_post_locales
+      WHERE id IN (
+        SELECT id
+        FROM (
+          SELECT id,
+                 ROW_NUMBER() OVER (
+                   PARTITION BY post_id
+                   ORDER BY created_at DESC, id DESC
+                 ) AS rnum
+          FROM discourse_translator_post_locales
+        ) t
+        WHERE t.rnum > 1
+      )
+    SQL
+
+    execute <<~SQL
+      DROP INDEX IF EXISTS index_discourse_translator_post_locales_on_post_id
+    SQL
+
+    execute <<~SQL
+      CREATE UNIQUE INDEX CONCURRENTLY index_discourse_translator_post_locales_on_post_id
+      ON discourse_translator_post_locales (post_id)
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end