Added code for backend glue

backend/internal/models/models.go

@@ -0,0 +1,30 @@
+package models
+
+import "time"
+
+type User struct {
+	ID    int64
+	Email string
+}
+
+type Conversation struct {
+	ID       int64
+	OwnerID  int64
+	Title    string
+	Created  time.Time
+}
+
+type Node struct {
+	ID             int64
+	ConversationID int64
+	AuthorKind     string // 'user' | 'assistant'
+	Content        string
+	Created        time.Time
+}
+
+type Branch struct {
+	ID             int64
+	ConversationID int64
+	Name           string
+	HeadNodeID     int64
+}