Harden restore-volumes.sh against silent restore truncation and incomplete-download skips

- BUG 1: Add error checking after tar extraction for both LOCAL and REMOTE-CACHE branches
  - Check exit status of tar -I zstd -xf commands
  - Print error to stderr and exit non-zero on failure
  - Prevents silent truncation where corrupt/incomplete backup extracts partial data
  - Mirrors existing remote-STREAM branch error handling
- BUG 2: Fix REMOTE branch to resume incomplete aria2c downloads
  - Check for presence of <file>.aria2 control file as incomplete signal
  - aria2c -c continues/resumes download when .aria2 file exists
  - Only skip download when file exists AND no .aria2 control file remains
  - aria2 deletes .aria2 sidecar on successful completion, making it a reliable signal
- Maintain all existing flags: aria2c -c -Z -x8 -j8 -s8 -d
- Preserve reth guard logic and static-file offload behavior unchanged

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

restore-volumes.sh

@@ -117,17 +117,26 @@ while IFS= read -r key; do
 	    echo "No /backup cache: streaming + extracting $newest_file directly"
 	    curl --ipv4 -# "${remote_source}${newest_file}" | zstd -d | tar -xf - --keep-directory-symlink -C /
 	    if [ $? -ne 0 ]; then
-		echo "Error processing $newest_file"
+		echo "Error processing $newest_file" >&2
 		exit 1
 	    fi
 	else
-	    if [ ! -e "$backup_dir/$(basename "$newest_file")" ]; then
+	    backup_file="$backup_dir/$(basename "$newest_file")"
+	    if [ ! -e "$backup_file" ] || [ -e "${backup_file}.aria2" ]; then
 		aria2c -c -Z -x8 -j8 -s8 -d "$backup_dir" "${remote_source}${newest_file}"
 	    fi
-	    tar -I zstd -xf "$backup_dir/$(basename "$newest_file")" --keep-directory-symlink -C /
+	    tar -I zstd -xf "$backup_file" --keep-directory-symlink -C /
+	    if [ $? -ne 0 ]; then
+		echo "Error processing $newest_file" >&2
+		exit 1
+	    fi
 	fi
     else
 	tar -I zstd -xf "$newest_file" --keep-directory-symlink -C /
+	    if [ $? -ne 0 ]; then
+		echo "Error processing $newest_file" >&2
+		exit 1
+	    fi
     fi
     echo "Backup '$newest_file' restored"
 done <<< "$keys"