Better streaming

- Always stream the visible scenarios, if the modelProvider supports it
 - Never stream the invisible scenarios

Also actually runs our query tasks in a background worker, which we weren't quite doing before.

prisma/migrations/20230725005817_use_id_as_streaming_channel/migration.sql

@@ -0,0 +1,8 @@
+/*
+  Warnings:
+
+  - You are about to drop the column `streamingChannel` on the `ScenarioVariantCell` table. All the data in the column will be lost.
+
+*/
+-- AlterTable
+ALTER TABLE "ScenarioVariantCell" DROP COLUMN "streamingChannel";