fix: add DNS propagation delay and frontend error feedback for tunnel recreation

- apps/api/src/services/tunnel.py: add 2-second sleep after discovering the
  tunnel URL to allow Cloudflare DNS edge propagation before returning
- apps/web/src/hooks/use-instance-actions.ts: show alert() with the backend
  error message when recreate tunnel fails, instead of silently swallowing
  errors

Quality gates: ruff clean, tsc clean

apps/api/src/services/tunnel.py

@@ -169,6 +169,9 @@ def start_tunnel(
             f"Exit code: {exit_code}. Logs:\n{combined_logs[-3000:]}"
         )
 
+    # Wait a moment for Cloudflare DNS edge to propagate the new tunnel subdomain
+    __import__("time").sleep(2)
+
     logger.info(
         "Tunnel %s started for %s → %s (%s)",
         tunnel_name,