PERF: optimize translation progress dashboard with batch query (#34861)

Replace individual per-locale queries with a single query that
calculates completion progress for all locales at once.

This commit also removes methods off the `base_candidate` (used for
Categories and Topics as well) which are not used any more, as the
progress dashboard only shows progress for posts.

t/162846/4

---------

Co-authored-by: Nat <natalie.tay@discourse.org>

db/migrate/20250925182715_add_index_to_posts_locale.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+class AddIndexToPostsLocale < ActiveRecord::Migration[8.0]
+  disable_ddl_transaction!
+
+  def up
+    remove_index :posts, :locale, algorithm: :concurrently, if_exists: true
+    add_index :posts, :locale, algorithm: :concurrently
+  end
+
+  def down
+    remove_index :posts, :locale, algorithm: :concurrently, if_exists: true
+  end
+end