refactor(tenant): drop legacy plugin_installations, migrate monitoring to desktop_clients

Hard cutover from the browser-extension plugin flow to desktop clients:
remove plugin_installations/plugin_sessions tables and related service,
handler, router, and generated model code; migrate monitoring quotas
and collector types to desktop_clients (UUID primary_client_id);
recreate platform_access_snapshots keyed by client_id; update dev-seed
and callback types accordingly; mark legacy design docs as historical.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/migrations/20260402140000_create_media_publisher_tables.down.sql

@@ -1,6 +1,4 @@
 DROP TABLE IF EXISTS publish_records;
 DROP TABLE IF EXISTS publish_batches;
-DROP TABLE IF EXISTS plugin_sessions;
-DROP TABLE IF EXISTS plugin_installations;
 DROP TABLE IF EXISTS platform_accounts;
 DROP TABLE IF EXISTS media_platforms;