feat: add initial backend and frontend structure with models, DTOs, and WebSocket events

apps/backend/prisma/models/message.prisma

@@ -0,0 +1,24 @@
+// Message model
+
+enum MessageRole {
+  user
+  assistant
+  system
+}
+
+model Message {
+  id              String      @id @default(uuid())
+  role            MessageRole
+  content         String
+  tokensUsed      Int?
+  model           String?
+  metadata        Json?
+  createdAt       DateTime    @default(now())
+
+  conversationId  String
+  conversation    Conversation @relation(fields: [conversationId], references: [id], onDelete: Cascade)
+
+  @@index([conversationId])
+  @@index([createdAt])
+  @@index([conversationId, createdAt])
+}