feat: add multilingual subtitle tracks

prisma/schema.prisma

@@ -71,6 +71,7 @@ model Lesson {
 
   progress    Progress[]
   bookmarks   Bookmark[]
+  subtitles    SubtitleTrack[]
   quiz         Quiz?
   quizAttempts  QuizAttempt[]
 
@@ -79,6 +80,22 @@ model Lesson {
   @@index([filePath])
 }
 
+model SubtitleTrack {
+  id        String   @id @default(cuid())
+  lessonId  String
+  lesson    Lesson   @relation(fields: [lessonId], references: [id], onDelete: Cascade)
+  src       String
+  lang      String
+  label     String
+  format    String
+  isDefault Boolean  @default(false)
+  createdAt DateTime @default(now())
+  updatedAt DateTime @updatedAt
+
+  @@unique([lessonId, lang, src])
+  @@index([lessonId])
+}
+
 model Progress {
   id          String   @id @default(cuid())
   userId      String   @default("local-user")