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/login.html

@@ -3,13 +3,13 @@
 <head>
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
-<title>Log in - bike-admin</title>
+<title>Log in - volundr</title>
 <link rel="icon" href="/static/favicon.svg" type="image/svg+xml">
 <link rel="stylesheet" href="/static/main.css">
 </head>
 <body class="auth">
 <main class="container auth-card">
-  <h1 style="margin-top:0">bike-admin</h1>
+  <h1 style="margin-top:0">volundr</h1>
   <p class="muted">Log in to manage your bikes.</p>
 
   {% if error %}