Organize backend into product executable and shared Go modules

backend/shared/database/repository/builder/unwind.go

@@ -0,0 +1,23 @@
+package builder
+
+// UnwindOption is a functional option for configuring the $unwind stage.
+type UnwindOption func(*UnwindOpts)
+
+type UnwindOpts struct {
+	PreserveNullAndEmptyArrays bool
+	IncludeArrayIndex          string
+}
+
+// WithPreserveNullAndEmptyArrays tells $unwind to keep docs where the array is null/empty.
+func WithPreserveNullAndEmptyArrays() UnwindOption {
+	return func(o *UnwindOpts) {
+		o.PreserveNullAndEmptyArrays = true
+	}
+}
+
+// WithIncludeArrayIndex adds an array‐index field named idxField to each unwound doc.
+func WithIncludeArrayIndex(idxField string) UnwindOption {
+	return func(o *UnwindOpts) {
+		o.IncludeArrayIndex = idxField
+	}
+}