feat: configurable unused images threshold with dashboard warning

- Add image_prune_threshold_mb setting (default 1024 MB)
- Add GET /api/docker/unused-images endpoint returning unused image count, size, and threshold status
- Dashboard shows amber warning banner when unused project images exceed threshold
- Banner links to settings page for pruning, shows count and human-readable size (MB/GB)
- Threshold configurable in Docker Image Cleanup section of settings
- DB migration + schema for image_prune_threshold_mb

web/src/routes/settings/+page.svelte

@@ -23,6 +23,7 @@
 	let baseVolumePath = $state('');
 	let notificationUrl = $state('');
 	let staleThresholdDays = $state('7');
+	let imagePruneThresholdMb = $state('1024');
 
 
 	// Proxy provider state.
@@ -128,6 +129,7 @@
 			baseVolumePath = settings.base_volume_path ?? '';
 			notificationUrl = settings.notification_url ?? '';
 			staleThresholdDays = String(settings.stale_threshold_days ?? 7);
+			imagePruneThresholdMb = String(settings.image_prune_threshold_mb ?? 1024);
 			proxyProvider = settings.proxy_provider ?? 'npm';
 			wildcardDns = settings.wildcard_dns ?? true;
 			dnsProvider = settings.dns_provider ?? '';
@@ -157,6 +159,7 @@
 				base_volume_path: baseVolumePath.trim(), notification_url: notificationUrl.trim(),
 				proxy_provider: proxyProvider,
 				stale_threshold_days: Math.max(1, parseInt(staleThresholdDays, 10) || 7),
+				image_prune_threshold_mb: Math.max(0, parseInt(String(imagePruneThresholdMb), 10) || 0),
 				wildcard_dns: wildcardDns,
 				dns_provider: wildcardDns ? '' : dnsProvider,
 				cloudflare_zone_id: cloudflareZoneId
@@ -363,6 +366,16 @@
 			<div class="mt-6 border-t border-[var(--border-primary)] pt-4">
 				<h3 class="mb-1 text-sm font-semibold text-[var(--text-primary)]">{$t('settings.dockerCleanup')}</h3>
 				<p class="mb-3 text-xs text-[var(--text-tertiary)]">{$t('settings.dockerCleanupHelp')}</p>
+				<div class="mb-3 max-w-xs">
+					<FormField
+						label={$t('settings.pruneThreshold')}
+						name="imagePruneThresholdMb"
+						type="number"
+						bind:value={imagePruneThresholdMb}
+						placeholder="1024"
+						helpText={$t('settings.pruneThresholdHelp')}
+					/>
+				</div>
 				<button
 					type="button"
 					onclick={() => { showPruneConfirm = true; }}