Add pundit-matchers gem and enhance indexed post handling

- Added `pundit-matchers` gem to improve policy testing capabilities.
- Updated `BlobsController` to support a new "tiny" size option for avatars.
- Enhanced `IndexablePostsHelper` with a `show_path` method for different postable types.
- Refactored `IndexedPost` model to include methods for retrieving artist information and external links.
- Modified `Domain::E621::Post` model to initialize `tags_array` as a hash.
- Updated views for indexed posts to support new display formats (gallery and table).
- Improved test coverage with new user factory and updated specs for controller and job behaviors.

Gemfile

@@ -90,6 +90,7 @@ group :test do
   gem "shoulda-matchers"
   gem "factory_bot_rails"
   gem "parallel_tests"
+  gem "pundit-matchers", "~> 4.0"
 end
 
 gem "xdiff", path: "/gems/xdiff-rb"