feat: move n8n to dev stack under n8n.dev.pivoine.art

- Moved n8n service from standalone stack to dev/compose.yaml
- Updated hostname from n8n.pivoine.art to n8n.dev.pivoine.art
- Updated environment variables to use DEV_N8N_ prefix
- Updated restic backup volume reference (n8n_n8n_data -> dev_n8n_data)
- Removed standalone n8n/compose.yaml
- Container name changes from n8n_app to dev_n8n
- Volume name changes from n8n_n8n_data to dev_n8n_data

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

restic/compose.yaml

@@ -107,7 +107,7 @@ volumes:
     name: tandoor_mediafiles
     external: true
   backup_n8n_data:
-    name: n8n_n8n_data
+    name: dev_n8n_data
     external: true
   backup_filestash_data:
     name: stash_filestash_data