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>

clone-backup.sh

@@ -298,6 +298,7 @@ cleanup_all_ports() {
 # Transfer backup file using screen method with better error handling
 transfer_backup() {
     local key=$1
+    local compose_file=$2
     local volume_name="rpc_$key"
     
     # Find the newest restorable backup file (skip near-empty artifacts)
@@ -309,6 +310,13 @@ transfer_backup() {
             echo "Note: no backup for secrets/jwt volume $volume_name (regenerated at create-node), skipping"
             return 2
         fi
+        # Check if this volume is optional or ephemeral - if so, skip gracefully
+        if [[ -n "$compose_file" && -f "$compose_file" ]]; then
+            if is_ephemeral_volume_key "$key" "$compose_file" || is_optional_volume_key "$key" "$compose_file"; then
+                echo "Note: no backup for optional/ephemeral volume $volume_name (will be created empty at destination), skipping"
+                return 2
+            fi
+        fi
         echo "Warning: No restorable backup file found for $volume_name, skipping"
         return 1
     fi
@@ -502,6 +510,7 @@ LISTENER
 # Fallback: Direct SSH pipe
 transfer_backup_ssh() {
     local key=$1
+    local compose_file=$2
     local volume_name="rpc_$key"
     
     # Find the newest restorable backup file (skip near-empty artifacts)
@@ -513,6 +522,13 @@ transfer_backup_ssh() {
             echo "Note: no backup for secrets/jwt volume $volume_name (regenerated at create-node), skipping"
             return 2
         fi
+        # Check if this volume is optional or ephemeral - if so, skip gracefully
+        if [[ -n "$compose_file" && -f "$compose_file" ]]; then
+            if is_ephemeral_volume_key "$key" "$compose_file" || is_optional_volume_key "$key" "$compose_file"; then
+                echo "Note: no backup for optional/ephemeral volume $volume_name (will be created empty at destination), skipping"
+                return 2
+            fi
+        fi
         echo "Warning: No restorable backup file found for $volume_name, skipping"
         return 1
     fi
@@ -642,7 +658,7 @@ main() {
 
     for key in $keys; do
         # Try nc method first
-        transfer_backup "$key"
+        transfer_backup "$key" "$dir/$1.yml"
         rc=$?
 
         if [[ $rc -eq 2 ]]; then
@@ -650,7 +666,7 @@ main() {
             skipped_volumes="$skipped_volumes $key"
         elif [[ $rc -ne 0 ]]; then
             echo "NC transfer failed, trying direct SSH..."
-            transfer_backup_ssh "$key"
+            transfer_backup_ssh "$key" "$dir/$1.yml"
             rc=$?
 
             if [[ $rc -eq 0 ]]; then