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).

app/templates/dashboard.html

@@ -1,5 +1,5 @@
 {% extends "base.html" %}
-{% block title %}Dashboard - bike-admin{% endblock %}
+{% block title %}Dashboard - volundr{% endblock %}
 {% block content %}
 <div class="page-head">
   <h1>Your bikes</h1>