Add multi-adapter test support: SQLite + MySQL via Gitea Actions and rake task

- .gitea/workflows/test.yml: two parallel CI jobs (SQLite and MySQL),
  each generates its own database.yml inline and runs the test suite
- lib/tasks/test_multi_db.rake: `rails test:all_adapters` runs both
  adapters sequentially using DATABASE_URL to switch at runtime

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/tasks/test_multi_db.rake

@@ -0,0 +1,50 @@
+namespace :test do
+  desc "Run Rails tests against all supported database adapters (SQLite, MySQL)"
+  task :all_adapters do
+    # DATABASE_URL overrides the adapter from database.yml at runtime.
+    # MySQL requires the mysql2 gem: bundle install --with mysql
+    adapters = {
+      "SQLite" => {
+        "DATABASE_URL" => "sqlite3:db/test.sqlite3"
+      },
+      "MySQL" => {
+        "DATABASE_URL" => format(
+          "mysql2://%s:%s@%s/%s",
+          ENV.fetch("DATABASE_USERNAME", "root"),
+          ENV.fetch("DATABASE_PASSWORD", ""),
+          ENV.fetch("DATABASE_HOST", "127.0.0.1"),
+          ENV.fetch("DATABASE_NAME", "fixin_test")
+        )
+      }
+    }
+
+    failed = []
+
+    adapters.each do |name, extra_env|
+      puts "\n#{"=" * 60}"
+      puts "  Running tests with #{name}"
+      puts "=" * 60
+
+      env = ENV.to_h.merge("RAILS_ENV" => "test").merge(extra_env)
+
+      # Reset test database; db:drop may fail on first run — that's fine
+      system(env, "bin/rails db:drop")
+      unless system(env, "bin/rails db:create db:schema:load")
+        failed << "#{name} (database setup)"
+        next
+      end
+
+      failed << name unless system(env, "bin/rails test")
+    end
+
+    puts "\n#{"=" * 60}"
+    if failed.any?
+      puts "  FAILED: #{failed.join(", ")}"
+      puts "=" * 60
+      exit 1
+    else
+      puts "  All adapters passed!"
+      puts "=" * 60
+    end
+  end
+end