feat: cleanup stale task containers at validator startup

When the validator worker starts, it now cleans up any orphaned task
containers from previous runs. This prevents container accumulation
after crashes or restarts.

Uses the container backend's cleanup() method which filters containers
by the challenge_id label.

Author: echobt

src/validator_worker.rs

@@ -89,6 +89,23 @@ impl ValidatorWorker {
             .await
             .context("Failed to create container backend")?;
 
+        // Cleanup stale task containers from previous runs
+        // This prevents orphaned containers from accumulating after crashes/restarts
+        match container_backend.cleanup(&challenge_id).await {
+            Ok(count) => {
+                if count > 0 {
+                    info!(
+                        "Cleaned up {} stale task containers from previous runs",
+                        count
+                    );
+                }
+            }
+            Err(e) => {
+                warn!("Failed to cleanup stale containers at startup: {}", e);
+                // Continue anyway - stale containers are not fatal
+            }
+        }
+
         Ok(Self {
             platform_url,
             challenge_id,