fix(observability): address final review findings

Critical fixes:
- Fix StaleContainer frontend type to match nested backend response shape
- Guard ContainerID[:12] slice against empty/short IDs in ListAllProxies

High-priority fixes:
- Support comma-separated severity/source in event log filtering (IN clause)
- Eliminate N+1 queries in ListAllProxies and FindStaleInstances (pre-load maps)
- Stop leaking internal error messages to API clients (use slog + generic msgs)

web/src/lib/components/StaleContainerCard.svelte

@@ -21,7 +21,7 @@
 	);
 
 	const displayName = $derived(
-		`${container.project_name}-${container.stage_name}-${container.image_tag}`
+		`${container.project_name}-${container.stage_name}-${container.instance.image_tag}`
 	);
 
 	function formatDate(iso: string): string {
@@ -59,14 +59,14 @@
 	<div class="mt-3 flex flex-wrap items-center gap-x-4 gap-y-1.5 text-xs text-[var(--text-secondary)]">
 		<span class="inline-flex items-center gap-1">
 			<IconTag size={12} />
-			{container.image_tag}
+			{container.instance.image_tag}
 		</span>
 		<span class="inline-flex items-center gap-1">
 			<IconClock size={12} />
-			{$t('stale.lastAlive')}: {formatDate(container.last_alive_at)}
+			{$t('stale.lastAlive')}: {formatDate(container.instance.last_alive_at)}
 		</span>
 		<span class="rounded bg-[var(--surface-card-hover)] px-1.5 py-0.5 font-mono text-[10px]">
-			{container.status}
+			{container.instance.status}
 		</span>
 	</div>
 
@@ -75,7 +75,7 @@
 		<button
 			type="button"
 			disabled={cleaning}
-			onclick={() => oncleanup(container.id)}
+			onclick={() => oncleanup(container.instance.id)}
 			class="inline-flex items-center gap-1.5 rounded-lg border border-[var(--color-danger)] px-3 py-1.5 text-xs font-medium text-[var(--color-danger)] transition-colors hover:bg-[var(--color-danger-light)] disabled:opacity-50 active:animate-press"
 		>
 			<IconTrash size={14} />