Add safe manual instance rebalancing

src/routes/hosts/+page.svelte

@@ -49,7 +49,7 @@
 <svelte:head><title>Hosts · Electric Cherry</title></svelte:head>
 <section class="page">
     <SectionHeader title="Hosts">
-        {#snippet actions()}{#if canManage}<Button variant="primary" onclick={() => addOpen = true}>Add host</Button>{/if}{/snippet}
+        {#snippet actions()}<Button href="/capacity?rebalance=1">Rebalance</Button>{#if canManage}<Button variant="primary" onclick={() => addOpen = true}>Add host</Button>{/if}{/snippet}
     </SectionHeader>
     {#if error}<p role="alert" class="ui-error">{error} Showing the last loaded hosts. <Button variant="ghost" size="sm" onclick={() => load()}>Retry</Button></p>{/if}
     {#if hosts.length}