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

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+[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

deploy/env.production.example

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+# bike_admin production environment (copied to /opt/bike_admin/.env on the server)
+# All values are BIKE_ADMIN_-prefixed. This file is NOT committed (see .gitignore).
+
+# --- Required -------------------------------------------------
+BIKE_ADMIN_USERNAME=jake
+BIKE_ADMIN_PASSWORD=change-me-to-something-strong
+
+# --- Required in production -----------------------------------
+# Secret used to derive the CSRF / session tokens. Generate with:
+#   python3 -c "import secrets;print(secrets.token_urlsafe(32))"
+BIKE_ADMIN_SECRET_KEY=
+
+# Adds the Secure flag to the session cookie (we're served over HTTPS via Caddy).
+BIKE_ADMIN_IS_PRODUCTION=true
+
+# Bind to all interfaces so the hermodr reverse proxy (on another host) can reach it.
+# The app trusts X-Forwarded-* headers from the proxy (BIKE_ADMIN_TRUST_PROXY defaults true).
+BIKE_ADMIN_BIND_HOST=0.0.0.0
+BIKE_ADMIN_BIND_PORT=8000
+
+# --- Optional fine-tuning (defaults shown) ---------------------
+# BIKE_ADMIN_SERVICE_DUE_SOON_MILES=200   # service "book soon" lead time
+# BIKE_ADMIN_RENEWAL_DUE_SOON_DAYS=60     # MOT/insurance "due soon" lead time
+# BIKE_ADMIN_RENEWAL_MONTHS=12            # MOT/insurance validity (months)
+# BIKE_ADMIN_DATA_DIR=/opt/bike_admin/data