Added code for backend glue

backend/internal/glue/branches.go

@@ -0,0 +1,12 @@
+package glue
+
+import (
+	"context"
+	"mind/internal/models"
+)
+
+type ForkReq struct { ConversationID int64; Name string; HeadNodeID int64 }
+
+func (g *Glue) ForkBranch(ctx context.Context, fr ForkReq) (models.Branch, error) {
+	return g.repo.CreateOrGetBranch(ctx, fr.ConversationID, fr.Name, fr.HeadNodeID)
+}