feat: add Gemastik A/D control panel (web UI for receiver)

- FastAPI app at panel/ proxying receiver API server-side (admin creds stay server-side)
- Login-protected dashboard: SLA status, rotate flag, restart/rollback/activate/deactivate, SSH creds, command history
- Runs as systemd service gemastik-panel.service on :18081
- Published at https://panel.gemastik.imrnes.team via Traefik

.gitignore

@@ -8,3 +8,4 @@ receiver/lib64
 receiver/pyenv.cfg
 receiver/include
 receiver/pwntools-docreceiver/.venv/
+panel/.env