Unexpected change of JWT Key

Maybe because Deno Deploy is not running on a single server, the jwt key I created will change from time to time, resulting in unexpected jwt token invalidation. Tokens generated with the old key will be invalid after the key is updated.
```ts
export const jwtKey = await crypto.subtle.generateKey(
{ name: "HMAC", hash: "SHA-512" },
true,
["sign", "verify"],
);
````
Not sure if there is any specific way to solve it.

Start with the crypto.subtle.generateKey snippet in the issue and investigate how generated keys behave across Deno Deploy instances and restarts. Reproduce whether tokens signed with an earlier key become unverifiable, then document the supported way to retain or share the JWT key, or clearly record the platform limitation.

Written by the indexing model from the issue text.