fix(tag): prevent dupl. tags on concurrent inserts

Currently, creating tags concurrently with different case
letters can raise a PostgreSQL unique constraint error
(index_tags_on_name_lower_btree).

This commit normalizes tag names to lowercase and uses
first_or_create! with a rescue for RecordNotUnique to
ensure that only one tag is created even under race conditions.

A test is added to simulate concurrent tag creation
and verify that only a single tag exists and no errors occur.

app/models/tag.rb

@@ -112,8 +112,14 @@ class Tag < ApplicationRecord
       names = Array(name_or_names).map { |str| [normalize(str), str] }.uniq(&:first)
 
       names.map do |(normalized_name, display_name)|
-        tag = matching_name(normalized_name).first || create(name: normalized_name,
-                                                             display_name: display_name.gsub(HASHTAG_INVALID_CHARS_RE, ''))
+        tag = begin
+          matching_name(normalized_name).first_or_create!(
+            name: normalized_name,
+            display_name: display_name.gsub(HASHTAG_INVALID_CHARS_RE, '')
+          )
+        rescue ActiveRecord::RecordNotUnique
+          find_normalized(normalized_name)
+        end
 
         yield tag if block_given?
 

spec/models/tag_spec.rb

@@ -261,6 +261,27 @@ RSpec.describe Tag do
     end
   end
 
+  describe '.find_or_create_by_names_race_condition' do
+    it 'handles simultaneous inserts of the same tag in different cases without error' do
+      tag_name_upper = 'Rails'
+      tag_name_lower = 'rails'
+
+      threads = []
+
+      2.times do |i|
+        threads << Thread.new do
+          Tag.find_or_create_by_names(i.zero? ? tag_name_upper : tag_name_lower)
+        end
+      end
+
+      threads.each(&:join)
+
+      tags = Tag.where('lower(name) = ?', tag_name_lower.downcase)
+      expect(tags.count).to eq(1)
+      expect(tags.first.name.downcase).to eq(tag_name_lower.downcase)
+    end
+  end
+
   describe '.search_for' do
     it 'finds tag records with matching names' do
       tag = Fabricate(:tag, name: 'match')