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>

arc/reth/arc-mainnet-reth-archive-trace.yml

@@ -160,6 +160,9 @@ volumes:
   arc-mainnet-reth-archive-trace-execution:
   arc-mainnet-reth-archive-trace-sockets:
 
+x-ephemeral-volumes:
+  - arc-mainnet-reth-archive-trace-sockets
+
 x-upstreams:
   - id: $${ID}
     labels:

arc/reth/arc-mainnet-reth-pruned-trace.yml

@@ -161,6 +161,9 @@ volumes:
   arc-mainnet-reth-pruned-trace-execution:
   arc-mainnet-reth-pruned-trace-sockets:
 
+x-ephemeral-volumes:
+  - arc-mainnet-reth-pruned-trace-sockets
+
 x-upstreams:
   - id: $${ID}
     labels:

arc/reth/arc-testnet-reth-archive-trace.yml

@@ -160,6 +160,9 @@ volumes:
   arc-testnet-reth-archive-trace-execution:
   arc-testnet-reth-archive-trace-sockets:
 
+x-ephemeral-volumes:
+  - arc-testnet-reth-archive-trace-sockets
+
 x-upstreams:
   - id: $${ID}
     labels:

arc/reth/arc-testnet-reth-pruned-trace.yml

@@ -161,6 +161,9 @@ volumes:
   arc-testnet-reth-pruned-trace-execution:
   arc-testnet-reth-pruned-trace-sockets:
 
+x-ephemeral-volumes:
+  - arc-testnet-reth-pruned-trace-sockets
+
 x-upstreams:
   - id: $${ID}
     labels:

arc/reth/arc-testnet-reth-pruned.yml

@@ -161,6 +161,9 @@ volumes:
   arc-testnet-reth-pruned-execution:
   arc-testnet-reth-pruned-sockets:
 
+x-ephemeral-volumes:
+  - arc-testnet-reth-pruned-sockets
+
 x-upstreams:
   - id: $${ID}
     labels: