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/api/onboarding/complete/route.ts

@@ -0,0 +1,17 @@
+import { auth } from '@/lib/auth';
+import { db } from '@/lib/db';
+import { apiErrors, successResponse } from '@/lib/api-response';
+
+export async function POST() {
+    const session = await auth();
+    if (!session?.user?.id) {
+        return apiErrors.unauthorized();
+    }
+
+    await db.user.update({
+        where: { id: session.user.id },
+        data: { onboardingCompletedAt: new Date() },
+    });
+
+    return successResponse({ completed: true });
+}