feat(cirkel): remove the old Cirkels pull-protocol; Cirkels is now AP-only
Phase 4 of the Cirkels-on-AP rework. Removes the legacy custom-Ed25519 pull
protocol and everything around it: /.klonkt/* (federation.js), CircleService +
CircleFederation, the 15-min sync loop, the /admin/circle UI, the remote_posts
union in /cirkel and the /cirkel/:id local-reading page. Tenancy is retired
(getTenancy always 'solo'); the Solo|Circle mode picker is gone. /cirkel is now
purely the auto-boosted (featured) feed. The circle_links/remote_* tables are
kept as dead data. A scripts/migrate-circles.mjs converts existing circle_links
to AP follows with auto-boost (run per instance).
Co-Authored-By: Claude <noreply@…>