fix: allow notification nonce reuse per user

prisma/models/user.prisma

@@ -24,7 +24,7 @@ model User {
 model Notification {
     id String @id @default(uuid())
 
-    nonce String? @unique
+    nonce String?
 
     userId String
     user   User   @relation(fields: [userId], references: [id])
@@ -35,4 +35,6 @@ model Notification {
     actions     String[]
 
     read Boolean @default(false)
+
+    @@unique([userId, nonce])
 }