Pin sqlite3 to ~> 1.4 for Rails 6.x compatibility

Rails 6.0/6.1 requires sqlite3 ~> 1.4, but without a constraint in the
gemspec, bundler resolves sqlite3 2.x on Ruby 3.0+ which is incompatible.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: flavorjones

marginalia.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |gem|
   gem.add_development_dependency "rake"
   gem.add_development_dependency "mysql2"
   gem.add_development_dependency "pg"
-  gem.add_development_dependency "sqlite3"
+  gem.add_development_dependency "sqlite3", "~> 1.4"
   gem.add_development_dependency "minitest"
   gem.add_development_dependency "mocha"
   gem.add_development_dependency "sidekiq"