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]>

prisma/schema.prisma

@@ -21,6 +21,7 @@ model User {
   password      String?   // Hashed password for email/password auth
   createdAt     DateTime  @default(now())
   updatedAt     DateTime  @updatedAt
+  onboardingCompletedAt DateTime?
 
   // Relations
   accounts           Account[]
@@ -70,12 +71,14 @@ enum DownloadEgressSource {
 enum VideoAssetKind {
   IMAGE
   VIDEO
+  AUDIO
 }
 
 enum VideoAssetProvider {
   R2_IMAGE
   YOUTUBE
   BUNNY
+  R2_AUDIO
 }
 
 model DownloadEgressEvent {