feat(backend): impl share expiry update

apps/backend/internal/nullable/time.go

@@ -0,0 +1,28 @@
+package nullable
+
+import (
+	"bytes"
+	"encoding/json"
+	"time"
+)
+
+// Time tracks whether a JSON field was set and stores an optional time value.
+type Time struct {
+	Time *time.Time
+	Set  bool
+}
+
+func (nt *Time) UnmarshalJSON(b []byte) error {
+	nt.Set = true
+	if bytes.Equal(bytes.TrimSpace(b), []byte("null")) {
+		nt.Time = nil
+		return nil
+	}
+
+	var t time.Time
+	if err := json.Unmarshal(b, &t); err != nil {
+		return err
+	}
+	nt.Time = &t
+	return nil
+}