Add experimental Standard.site support

[lylo]

Summary

Adds experimental Standard.site support for Pagecord blogs:

• separate Standard.site account/publication/document tables
• Bluesky connection UI using an app password for this first pass
• server-side AT record writes through a small client boundary
• publication/document sync jobs
• .well-known publication verification endpoint
• blog head tags for Standard.site publication and document records

Notes

This is intentionally marked draft because the connection layer uses Bluesky app passwords rather than full AT Protocol OAuth. The record models, jobs, verification endpoint, and metadata rendering are isolated so the auth path can be replaced later without changing the rest of the feature.

Verification

• bundle exec rubocop --cache false app/models/standard_site.rb app/models/standard_site app/jobs/standard_site app/controllers/app/settings/standard_site_controller.rb app/controllers/blogs/standard_site_publications_controller.rb app/helpers/blogs_helper.rb app/models/blog.rb app/models/post.rb test/controllers/app/settings/standard_site_controller_test.rb test/controllers/blogs/standard_site_publications_controller_test.rb test/models/standard_site
• bin/rails test