Add WordPress plugin ecosystem support

[AgentGoose32]

Summary

• add a WordPress plugin ecosystem adapter backed by the wordpress.org plugin API
• support registry/documentation/download URLs, WP-CLI install command, package lookup, package lists, recently updated packages, and version metadata
• seed wordpress.org/plugins as the default WordPress registry
• add model coverage for URLs, purl, package lists, metadata, downloads, and versions

Refs #660

Validation

• ruby -c app/models/ecosystem/wordpress.rb
• ruby -c test/models/ecosystem/wordpress_test.rb
• ruby -c db/seeds.rb
• git diff --check

Full Rails test execution is blocked locally because this repo lockfile requires Bundler 4.0.10 while the system Ruby/Bundler cannot satisfy it.