feat: add UserHighlight model to database schema

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Co-Authored-By: Claude <noreply@anthropic.com>

prisma/schema.prisma

@@ -33,6 +33,7 @@ model User {
   bookmarks       Bookmark[]
   chapterBookmarks ChapterBookmark[]
   highlights      Highlight[]
+  userHighlights  UserHighlight[]
   notes           Note[]
   chatMessages    ChatMessage[]
   chatConversations ChatConversation[]
@@ -245,6 +246,20 @@ model Highlight {
   @@index([verseId])
 }
 
+model UserHighlight {
+  id        String   @id @default(cuid())
+  userId    String
+  user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  verseId   String
+  color     String   @default("yellow") // yellow, orange, pink, blue
+  createdAt DateTime @default(now())
+  updatedAt DateTime @updatedAt
+
+  @@unique([userId, verseId])
+  @@index([userId])
+  @@index([verseId])
+}
+
 model Note {
   id          String   @id @default(uuid())
   userId      String