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.

app/composables/useUserSettings.ts

@@ -85,7 +85,7 @@ export const useUserSettings = async () => {
     });
 
     const effectiveSystemAssistants = computed(() => {
-        return localSystemAssistants.value || {};
+        return localSystemAssistants.value || {} as SystemAssistants;
     });
 
     const colorSchemeValue = computed(() => {