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/base.html

@@ -3,14 +3,14 @@
 <head>
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
-<title>{% block title %}bike-admin{% endblock %}</title>
+<title>{% block title %}volundr{% endblock %}</title>
 <link rel="icon" href="/static/favicon.svg" type="image/svg+xml">
 <link rel="stylesheet" href="/static/main.css">
 <script src="/static/htmx.min.js" defer></script>
 </head>
 <body>
 <header class="topbar">
-  <a class="brand" href="/">bike-admin</a>
+  <a class="brand" href="/">volundr</a>
   <nav class="nav">
     <a href="/bikes/new" class="btn btn-primary">+ Bike</a>
     <a href="/logout"