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/layout.tsx

@@ -0,0 +1,7 @@
+export default function OnboardingLayout({ children }: { children: React.ReactNode }) {
+    return (
+        <div className="min-h-screen bg-background flex items-center justify-center p-4">
+            {children}
+        </div>
+    );
+}