fix: change PushSubscription endpoint to VARCHAR for MySQL compatibility

MySQL cannot create unique constraints on TEXT fields without length.
Changed endpoint from @db.Text to @db.VarChar(512) to fix Prisma generation error.

prisma/schema.prisma

@@ -26,14 +26,14 @@ model User {
 model PushSubscription {
   id         String   @id @default(cuid())
   userId     String
-  endpoint   String   @db.Text
+  endpoint   String   @db.VarChar(512)
   p256dh     String   @db.Text
   auth       String   @db.Text
   createdAt  DateTime @default(now())
   updatedAt  DateTime @updatedAt
   user       User     @relation(fields: [userId], references: [id], onDelete: Cascade)
 
-  @@unique([userId, endpoint(255)])
+  @@unique([userId, endpoint])
   @@index([userId])
 }