Add optional/ephemeral volume notation for restore resilience

Introduce x-optional-volumes for beacon sidecars (prysm, nimbus, lighthouse,
teku, lodestar) that are checkpoint-sync capable and rebuild state quickly.
Mark reth sockets volumes as ephemeral (runtime IPC artifacts).

Script changes:
- volume-utils.sh: add is_optional_volume_key() function
- restore-volumes.sh: tolerate missing optional/ephemeral volume archives
- clone-backup.sh: skip missing optional/ephemeral volumes gracefully
- list-restorable.sh: only require archives for non-optional persistent volumes

This fixes the de-25 ethereum prysm incident (entire 1.1T restore failed
due to missing prysm archive) and arc-testnet reth sockets restore loops.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

restore-volumes.sh

@@ -168,6 +168,13 @@ while IFS= read -r key; do
     fi
 
     if [ -z "$newest_file" ]; then
+	# Check if this volume is optional or ephemeral - if so, warn and continue
+	if is_ephemeral_volume_key "$key" "$dir/$1.yml" || is_optional_volume_key "$key" "$dir/$1.yml"; then
+		echo "WARN: No backup found for volume 'rpc_$key' (volume is optional/ephemeral) - creating empty volume"
+		# Create empty volume and continue
+		mkdir -p "$data_dir"
+		continue
+	fi
 	echo "Error: No backup found for volume 'rpc_$key'"
 	exit 1
     fi