Rebrand to volundr; add deploy artifacts

- User-facing brand "bike-admin" -> "volundr" (page titles, top bar,
  login header, FastAPI title, README/AGENTS). Directory, remote,
  BIKE_ADMIN_* env prefix and bike_admin.db filename are unchanged.
- Add deploy/: systemd unit (bike-admin.service) and a production
  .env template (env.production.example).

deploy/bike_admin.service

@@ -0,0 +1,27 @@
+[Unit]
+Description=volundr (bike service-due + MOT/insurance tracker)
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+User=bike
+Group=bike
+WorkingDirectory=/opt/bike_admin
+# Leading `-` = optional: the app still boots (and prompts for a password)
+# if the .env has not been created yet.
+EnvironmentFile=-/opt/bike_admin/.env
+ExecStart=/opt/bike_admin/.venv/bin/uvicorn app.main:app \
+  --host 0.0.0.0 --port 8000 \
+  --forwarded-allow-ips 192.168.0.0/16 --proxy-headers
+Restart=on-failure
+RestartSec=2
+# Hardening
+NoNewPrivileges=true
+PrivateTmp=true
+ProtectSystem=full
+ProtectHome=true
+ReadWritePaths=/opt/bike_admin/data
+
+[Install]
+WantedBy=multi-user.target