docs(infra): add PostgreSQL tuning and systemd service documentation

- POSTGRESQL_TUNING.md: documents applied config (8GB shared_buffers,
  64MB work_mem, 8GB max_wal_size, SSD params)
- SYSTEMD_SERVICES.md: lists all production systemd services
- systemd/: versioned copies of all .service and .timer files
- .gitignore: ignore package-lock.json and backups/

.gitignore

@@ -80,3 +80,10 @@ node_modules/
 
 # Diagram images (served from static, too large for git)
 dashboard/static/diagrams/
+
+# Playwright / Node
+package-lock.json
+
+# Backups
+backups/
+