Fix is_expired to use <= for SuperBlockSize check matching Go

Go checks contentSize <= SuperBlockSize to detect empty volumes (no
needles). Rust used < which would incorrectly allow a volume with
exactly SuperBlockSize bytes (header only, no data) to proceed to
the TTL expiry check and potentially be marked as expired.

seaweed-volume/src/storage/volume.rs

@@ -1645,7 +1645,7 @@ impl Volume {
         if volume_size_limit == 0 {
             return false;
         }
-        if volume_size < SUPER_BLOCK_SIZE as u64 {
+        if volume_size <= SUPER_BLOCK_SIZE as u64 {
             return false;
         }
         let ttl_minutes = self.super_block.ttl.minutes();