feat: add general chat feature (without project/agent concept)

backend/prisma/schema.prisma

@@ -37,6 +37,7 @@ model User {
   knowledgeBases KnowledgeBase[]
   documents     Document[]
   adminLogs     AdminLog[]
+  generalConversations GeneralConversation[]
   
   @@index([email])
   @@index([status])
@@ -187,3 +188,42 @@ model AdminLog {
   @@index([action])
   @@map("admin_logs")
 }
+
+// ==================== 通用对话模块 ====================
+
+model GeneralConversation {
+  id            String   @id @default(uuid())
+  userId        String   @map("user_id")
+  title         String
+  modelName     String?  @map("model_name")
+  
+  createdAt     DateTime @default(now()) @map("created_at")
+  updatedAt     DateTime @default(now()) @updatedAt @map("updated_at")
+  deletedAt     DateTime? @map("deleted_at")
+  
+  user          User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  messages      GeneralMessage[]
+  
+  @@index([userId])
+  @@index([createdAt])
+  @@index([updatedAt])
+  @@map("general_conversations")
+}
+
+model GeneralMessage {
+  id             String   @id @default(uuid())
+  conversationId String   @map("conversation_id")
+  role           String
+  content        String   @db.Text
+  model          String?
+  metadata       Json?
+  tokens         Int?
+  
+  createdAt      DateTime @default(now()) @map("created_at")
+  
+  conversation   GeneralConversation @relation(fields: [conversationId], references: [id], onDelete: Cascade)
+  
+  @@index([conversationId])
+  @@index([createdAt])
+  @@map("general_messages")
+}