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>

ethereum/geth/ethereum-hoodi-geth-pruned-pebble-path--nimbus.yml

@@ -205,4 +205,5 @@ x-upstreams:
       generic:
         rpc:
           url: $${RPC_URL}
+x-optional-volumes: [ethereum-hoodi-geth-pruned-pebble-path_nimbus]
 ...

ethereum/geth/ethereum-hoodi-geth-pruned-pebble-path.yml

@@ -206,4 +206,5 @@ x-upstreams:
       generic:
         rpc:
           url: $${RPC_URL}
+x-optional-volumes: [ethereum-hoodi-geth-pruned-pebble-path_prysm]
 ...

ethereum/geth/ethereum-mainnet-geth-minimal-pebble-path.yml

@@ -210,4 +210,5 @@ x-upstreams:
       generic:
         rpc:
           url: $${RPC_URL}
+x-optional-volumes: [ethereum-mainnet-geth-minimal-pebble-path_prysm]
 ...

ethereum/geth/ethereum-mainnet-geth-pruned-leveldb-hash--nimbus--fireeth.yml

@@ -269,4 +269,5 @@ x-upstreams:
       generic:
         rpc:
           url: $${RPC_URL}
+x-optional-volumes: [ethereum-mainnet-geth-pruned-leveldb-hash_nimbus]
 ...

ethereum/geth/ethereum-mainnet-geth-pruned-pebble-path--nimbus--fireeth.yml

@@ -271,4 +271,5 @@ x-upstreams:
       generic:
         rpc:
           url: $${RPC_URL}
+x-optional-volumes: [ethereum-mainnet-geth-pruned-pebble-path_nimbus]
 ...

ethereum/geth/ethereum-mainnet-geth-pruned-pebble-path--nimbus.yml

@@ -205,4 +205,5 @@ x-upstreams:
       generic:
         rpc:
           url: $${RPC_URL}
+x-optional-volumes: [ethereum-mainnet-geth-pruned-pebble-path_nimbus]
 ...

ethereum/geth/ethereum-mainnet-geth-pruned-pebble-path.yml

@@ -206,4 +206,5 @@ x-upstreams:
       generic:
         rpc:
           url: $${RPC_URL}
+x-optional-volumes: [ethereum-mainnet-geth-pruned-pebble-path_prysm]
 ...

ethereum/geth/ethereum-sepolia-geth-pruned-pebble-path--nimbus.yml

@@ -205,4 +205,5 @@ x-upstreams:
       generic:
         rpc:
           url: $${RPC_URL}
+x-optional-volumes: [ethereum-sepolia-geth-pruned-pebble-path_nimbus]
 ...

ethereum/geth/ethereum-sepolia-geth-pruned-pebble-path.yml

@@ -206,4 +206,5 @@ x-upstreams:
       generic:
         rpc:
           url: $${RPC_URL}
+x-optional-volumes: [ethereum-sepolia-geth-pruned-pebble-path_prysm]
 ...