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>

list-restorable.sh

@@ -13,6 +13,20 @@
 
 dir="$(dirname "$0")"
 source "$dir/volume-utils.sh"
+
+# Get required persistent volume keys (all volumes minus ephemeral minus optional)
+get_required_volume_keys() {
+  local compose_file=$1
+  local key
+  
+  while IFS= read -r key; do
+    [[ -z "$key" ]] && continue
+    # Skip if ephemeral or optional
+    if ! is_ephemeral_volume_key "$key" "$compose_file" && ! is_optional_volume_key "$key" "$compose_file"; then
+      echo "$key"
+    fi
+  done < <(get_volume_keys "$compose_file")
+}
 registry_file="${dir}/compose_registry.json"
 backup_dir="/backup"
 
@@ -115,12 +129,14 @@ while IFS= read -r entry; do
         continue
     fi
     
-    # Check each volume for backup file
+    # Check each required volume for backup file
     all_backups_exist=true
     missing_volumes=()
     backup_info=()
     
-    while IFS= read -r volume; do
+    # Get required persistent volumes (excluding ephemeral and optional)
+    # Use process substitution to avoid subshell issues
+    while IFS= read -r volume < <(get_required_volume_keys "$compose_path"); do
         volume_name="rpc_${volume}"
         # jwt/secrets volumes are regenerated at create-node — never backed up by
         # design, so they must not gate restorability (kept plasma marked