feat(onboarding): add onboarding wizard flow

- Add `onboardingCompletedAt` field to User model + migration
- Add AUDIO/R2_AUDIO enum values to schema
- Gate dashboard behind onboarding check; redirect to /onboarding if not completed
- Add /onboarding page + layout with 5-step wizard (welcome, workspace, project, video info, notifications)
- Add POST /api/onboarding/complete endpoint to mark onboarding done

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

app/onboarding/page.tsx

@@ -0,0 +1,24 @@
+import { auth } from '@/lib/auth';
+import { db } from '@/lib/db';
+import { redirect } from 'next/navigation';
+import { OnboardingWizard } from './onboarding-wizard';
+
+export default async function OnboardingPage() {
+    const session = await auth();
+    if (!session?.user?.id) {
+        redirect('/login');
+    }
+
+    const user = await db.user.findUnique({
+        where: { id: session.user.id },
+        select: { onboardingCompletedAt: true, name: true, email: true },
+    });
+
+    if (user?.onboardingCompletedAt) {
+        redirect('/dashboard');
+    }
+
+    const userName = user?.name || user?.email?.split('@')[0] || 'there';
+
+    return <OnboardingWizard userName={userName} />;
+}