fix(db): generate schema types during prepare

[onmax]

Closes #792

Summary

PR #779 re-introduced if (!nuxt.options._prepare) guard around buildDatabaseSchema(), regressing PR #758 fix. This moves buildDatabaseSchema() outside the guard while keeping node_modules copying inside.

StackBlitz

	Link	Expected
Bug	nuxthub-792	❌ No schema.d.mts
Fix	nuxthub-792-fixed	✅ schema.d.mts generated

CLI Reproduction

git clone --depth 1 --filter=blob:none --sparse https://github.com/onmax/repros.git
cd repros && git sparse-checkout set nuxthub-792
cd nuxthub-792 && pnpm i && pnpm prepare && ls .nuxt/hub/db/

Verify fix

git sparse-checkout add nuxthub-792-fixed
cd ../nuxthub-792-fixed && pnpm i && pnpm prepare && ls .nuxt/hub/db/

The -fixed folder uses pnpm patch to apply the fix locally.

[vercel]

@onmax is attempting to deploy a commit to the NuxtLabs Team on Vercel.

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[pkg-pr-new]

npm i https://pkg.pr.new/@nuxthub/core@795

commit: 5e06986