Add owner context and harden ownership boundaries

backend/prisma/schema.prisma

@@ -97,7 +97,7 @@ model AudioAsset {
 }
 
 model ArtworkAsset {
-  userId        String?  @db.Uuid @map("user_id")
+  userId        String   @db.Uuid @map("user_id")
   id            String   @id @default(uuid()) @db.Uuid
   sha256        String
   storageKey    String   @unique @map("storage_key")
@@ -107,7 +107,7 @@ model ArtworkAsset {
   fileSizeBytes BigInt   @map("file_size_bytes")
   createdAt     DateTime @default(now()) @map("created_at")
   tracks        Track[]  @relation("TrackArtwork")
-  user          User?    @relation(fields: [userId], references: [id], onDelete: Cascade, onUpdate: Cascade)
+  user          User     @relation(fields: [userId], references: [id], onDelete: Cascade, onUpdate: Cascade)
 
   @@unique([userId, sha256])
   @@index([userId])