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

apps/backend/prisma/models/user.prisma

@@ -0,0 +1,25 @@
+// User model and related enums
+
+enum UserRole {
+  USER
+  ADMIN
+}
+
+model User {
+  id            String    @id @default(uuid())
+  email         String    @unique
+  username      String    @unique
+  passwordHash  String?
+  keycloakSub   String?   @unique
+  role          UserRole  @default(USER)
+  isActive      Boolean   @default(true)
+  createdAt     DateTime  @default(now())
+  updatedAt     DateTime  @updatedAt
+
+  characters    Character[]
+  conversations Conversation[]
+  importDocs    ImportDocument[]
+
+  @@index([email])
+  @@index([keycloakSub])
+}