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/20260402153000_repair_plugin_installations_schema.down.sql

@@ -1,4 +1 @@
--- No-op rollback.
--- This migration repairs databases that already applied an older variant of
--- 20260402140000 without plugin_installations. Dropping these objects on
--- rollback would break databases created from the current baseline migration.
+-- Legacy no-op rollback.