Implement Phase 10: deployment (Dockerfile, systemd, install script)

- Multi-stage Dockerfile: golang:1.25-alpine builder, alpine:3.21 runtime
  CGO_ENABLED=0, stripped binary, non-root user
- systemd: service unit (hardened), backup oneshot, daily timer (02:00 UTC)
- Install script: create user, dirs, config, install units
- Updated PROGRESS.md with all completed phases

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

deploy/systemd/eng-pad-server-backup.timer

@@ -0,0 +1,9 @@
+[Unit]
+Description=Daily Engineering Pad Server Backup
+
+[Timer]
+OnCalendar=*-*-* 02:00:00
+RandomizedDelaySec=300
+
+[Install]
+WantedBy=timers.target