feat(prisma/schema): add multilingual support for problem descriptions and solutions

BREAKING CHANGE:
- Removed `description` and `solution` fields from Problem model
- Added new models `ProblemDescription` and `ProblemSolution` with language support (EN/ZH)
- Updated seed data structure to support multilingual content
- Requires database migration and data migration from old structure

prisma/schema.prisma

@@ -47,9 +47,9 @@ enum Difficulty {
 model Problem {
   id           String               @id @default(cuid())
   displayId    Int                  @unique
-  title       String
+  titles       ProblemTitle[]
-  description String
+  descriptions ProblemDescription[]
-  solution    String
+  solutions    ProblemSolution[]
   difficulty   Difficulty           @default(EASY)
   published    Boolean              @default(false)
   userId       String
@@ -64,6 +64,41 @@ model Problem {
   @@index([difficulty])
 }
 
+enum Language {
+  en
+  zh
+}
+
+model ProblemTitle {
+  id        String   @id @default(cuid())
+  language  Language
+  content   String
+  problemId String
+  problem   Problem  @relation(fields: [problemId], references: [id], onDelete: Cascade)
+
+  @@unique([problemId, language])
+}
+
+model ProblemDescription {
+  id        String   @id @default(cuid())
+  language  Language
+  content   String
+  problemId String
+  problem   Problem  @relation(fields: [problemId], references: [id], onDelete: Cascade)
+
+  @@unique([problemId, language])
+}
+
+model ProblemSolution {
+  id        String   @id @default(cuid())
+  language  Language
+  content   String
+  problemId String
+  problem   Problem  @relation(fields: [problemId], references: [id], onDelete: Cascade)
+
+  @@unique([problemId, language])
+}
+
 enum EditorLanguage {
   c
   cpp