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/api/admin/pre-registrants/invite.post.js

@@ -4,7 +4,7 @@ import { Resend } from 'resend'
 import PreRegistration from '../../../models/preRegistration.js'
 import { connectDB } from '../../../utils/mongoose.js'
 
-const resend = new Resend(process.env.RESEND_API_KEY)
+const resend = new Resend(useRuntimeConfig().resendApiKey)
 
 export default defineEventHandler(async (event) => {
   await requireAdmin(event)