docs(backups): add OOP/Sandi Metz principles to Ruby notifier spec

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/superpowers/specs/2026-05-07-n8n-backup-monitor-design.md

@@ -34,6 +34,17 @@ n8n Workflow 2 (Reporter)
 
 ## Ruby Notifier Script (`notify_backup.rb`)
 
+### Design principles
+
+- **Sandi Metz OOP:** Small, single-purpose objects. Each class has one reason to change. Dependencies injected, not hardcoded. No class does more than one job.
+- **Self-documenting:** The script's header comment explains its purpose, all accepted arguments, exit behavior, and required ENV vars — enough for any future backup process to integrate without reading the implementation.
+- **Classes:**
+  - `BackupNotifier` — orchestrates the flow; the only public entry point
+  - `PayloadBuilder` — converts raw CLI args into a clean hash (formats bytes, validates status)
+  - `WebhookClient` — handles the HTTP POST with auth header; knows nothing about backup data
+  - `ByteFormatter` — single responsibility: raw integer → human-readable string
+  - CLI argument parsing at the top level (OptionParser), outside of classes
+
 **Location:** `~/.local/bin/notify_backup.rb` (on each machine that runs backups)
 
 **Called from backup scripts with named arguments:**