fix(helcim): tolerate empty response body on DELETE (204)

helcimFetch called response.json() unconditionally, which threw
"Unexpected end of JSON input" on Helcim's 204 No Content responses
(e.g. DELETE /subscriptions/:id). The error was silently swallowed by
the best-effort cancel path in cancel-subscription, masking cases where
the Helcim-side cancel actually succeeded.

server/utils/helcim.js

@@ -63,7 +63,13 @@ export async function helcimFetch(path, { method = 'GET', body, idempotencyKey,
     })
   }
 
-  return await response.json()
+  const text = await response.text()
+  if (!text.trim()) return null
+  try {
+    return JSON.parse(text)
+  } catch {
+    return null
+  }
 }
 
 // ---- Customers ----