feat(maintenance): add Docker build-cache prune action

Add an admin-only POST /api/docker/prune-build-cache endpoint plus a Settings > Maintenance danger-zone button to reclaim disk used by the Docker build cache (image + static-site builds), which previously grew unbounded with no UI lever. Prunes unused-only (all=false) so a warm cache is preserved for apps redeploying soon. Mirrors the existing prune-images vertical slice; full en/ru i18n parity.

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

@@ -6,7 +6,7 @@
   never within casual miss-click distance of general form fields.
 -->
 <script lang="ts">
-	import { getSettings, updateSettings, pruneImages } from '$lib/api';
+	import { getSettings, updateSettings, pruneImages, pruneBuildCache } from '$lib/api';
 	import FormField from '$lib/components/FormField.svelte';
 	import ConfirmDialog from '$lib/components/ConfirmDialog.svelte';
 	import Skeleton from '$lib/components/Skeleton.svelte';
@@ -18,6 +18,8 @@
 	let saving = $state(false);
 	let pruning = $state(false);
 	let showPruneConfirm = $state(false);
+	let pruningCache = $state(false);
+	let showPruneCacheConfirm = $state(false);
 
 	let staleThresholdDays = $state('7');
 	let imagePruneThresholdMb = $state('1024');
@@ -77,6 +79,18 @@
 		}
 	}
 
+	async function handlePruneBuildCache() {
+		pruningCache = true;
+		try {
+			const result = await pruneBuildCache();
+			toasts.success($t('settings.pruneCacheResult', { count: String(result.caches_deleted), mb: String(result.space_reclaimed_mb) }));
+		} catch (err) {
+			toasts.error(err instanceof Error ? err.message : $t('settings.pruneCacheFailed'));
+		} finally {
+			pruningCache = false;
+		}
+	}
+
 	$effect(() => { load(); });
 </script>
 
@@ -165,6 +179,26 @@
 							{/if}
 						</button>
 					</div>
+
+					<hr class="my-5 border-[var(--color-danger)]/20" />
+
+					<p class="text-sm text-[var(--text-secondary)]">{$t('settings.buildCacheCleanupHelp')}</p>
+
+					<div class="mt-4">
+						<button
+							type="button"
+							onclick={() => { showPruneCacheConfirm = true; }}
+							disabled={pruningCache}
+							class="inline-flex items-center gap-2 rounded-lg border border-[var(--color-danger)] bg-[var(--surface-card)] px-4 py-2 text-sm font-medium text-[var(--color-danger)] hover:bg-[var(--color-danger)] hover:text-white disabled:opacity-50 transition-colors"
+						>
+							{#if pruningCache}
+								<IconLoader size={16} />
+								{$t('settings.pruningCache')}
+							{:else}
+								{$t('settings.pruneBuildCache')}
+							{/if}
+						</button>
+					</div>
 				</div>
 			</div>
 		</div>
@@ -180,3 +214,13 @@
 	onconfirm={() => { showPruneConfirm = false; handlePruneImages(); }}
 	oncancel={() => { showPruneConfirm = false; }}
 />
+
+<ConfirmDialog
+	open={showPruneCacheConfirm}
+	title={$t('settings.pruneBuildCache')}
+	message={$t('settings.pruneBuildCacheConfirmMessage')}
+	confirmLabel={$t('settings.pruneBuildCache')}
+	confirmVariant="danger"
+	onconfirm={() => { showPruneCacheConfirm = false; handlePruneBuildCache(); }}
+	oncancel={() => { showPruneCacheConfirm = false; }}
+/>