Expose host repository stats API

[AgentGoose32]

Refs #680

Summary

• Adds JSON stats endpoints for global repos.ecosyste.ms data and individual hosts.
• Exposes counts plus top repositories and top owners by stars/repository counts, covering the API side of the requested instance/host detail pages.
• Adds controller coverage for both global and per-host stats responses.

Validation

• ruby -c app/controllers/api/v1/hosts_controller.rb
• ruby -c test/controllers/api/v1/hosts_controller_test.rb
• ruby -c config/routes.rb
• git diff --check

bundle exec ruby -Itest test/controllers/api/v1/hosts_controller_test.rb remains blocked locally because this checkout requires Bundler 4.0.10 from Gemfile.lock, which is not available in the system Ruby environment.