refactor(env): unify required-env validation through useRuntimeConfig

validate-env.js now reads all four required vars (MONGODB_URI, JWT_SECRET,
RESEND_API_KEY, HELCIM_API_TOKEN) from useRuntimeConfig() instead of mixing
direct process.env reads with a JWT-only special case. Mongoose and the six
Resend instantiations follow suit. Either bare or NUXT_-prefixed env names
are accepted, so Dokploy no longer needs duplicate entries.

server/utils/resend.js

@@ -1,6 +1,6 @@
 import { Resend } from "resend";
 
-const resend = new Resend(process.env.RESEND_API_KEY);
+const resend = new Resend(useRuntimeConfig().resendApiKey);
 
 /**
  * Send event registration confirmation email