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/components/Settings/SystemAssistants.vue

@@ -47,10 +47,11 @@ defineEmits(['navigate']);
         <div class="flex flex-col" v-for="(systemAssistant, key) in settings.systemAssistants" :key="key">
             <label :for="`slider-${key}`" class="flex justify-between gap-4 items-center">
                 <h4 class="case-capital">{{ key }}</h4>
-                <Slider :id="`slider-${key}`" :checked="systemAssistant.enabled" @click="toggle(String(key))" />
+                <Slider :id="`slider-${key}`" :checked="systemAssistant?.enabled ?? false"
+                    @click="toggle(String(key))" />
             </label>
             <div class="flex-1 justify-between gap-4 items-center">
-                <ModelSelector :providers="providers" :model-value="getModel(systemAssistant.modelId)"
+                <ModelSelector :providers="providers" :model-value="getModel(systemAssistant?.modelId)"
                     @update:model-value="(model) => updateModel(String(key), model)" />
             </div>
         </div>