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/(dashboard)/dashboard/page.tsx

@@ -14,6 +14,14 @@ export default async function DashboardPage({
         redirect('/login');
     }
 
+    const userOnboarding = await db.user.findUnique({
+        where: { id: session.user.id },
+        select: { onboardingCompletedAt: true },
+    });
+    if (!userOnboarding?.onboardingCompletedAt) {
+        redirect('/onboarding');
+    }
+
     const resolvedSearchParams = await searchParams;
     const { ws, sort, page: pageParam } = resolvedSearchParams || {};