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/lib/api.ts

@@ -438,6 +438,10 @@ export function pruneImages(): Promise<{ images_removed: number; space_reclaimed
 	return post<{ images_removed: number; space_reclaimed_mb: number }>('/api/docker/prune-images');
 }
 
+export function pruneBuildCache(): Promise<{ caches_deleted: number; space_reclaimed_mb: number }> {
+	return post<{ caches_deleted: number; space_reclaimed_mb: number }>('/api/docker/prune-build-cache');
+}
+
 export function testNpmConnection(data: { npm_url?: string; npm_email?: string; npm_password?: string }): Promise<{ status: string }> {
 	return post<{ status: string }>('/api/settings/npm/test', data);
 }