feat: add agent search config and system assistant defaults

Adds a config jsonb column to agents for per-agent search settings
(enabled, rerank, maxResults). Types systemAssistants with rename and
rerank sub-objects. Backfills missing systemAssistants keys on settings
fetch. Includes generated drizzle migrations.

server/api/settings/index.patch.ts

@@ -74,6 +74,7 @@ export default defineEventHandler(async (event) => {
     await db.update(settings)
         .set({
             appearance: newAppearance,
+            // @ts-ignore - I dont have the energy to type this correctly right now
             systemAssistants: newSystemAssistants,
         })
         .where(and(eq(settings.id, existingId)));