Convert backend from submodule to regular directory

Remove submodule tracking; backend is now a plain directory in the repo.
Also update deploy workflow: remove --recurse-submodules.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/api/templates/days.html

@@ -0,0 +1,46 @@
+{{define "title"}}Tage — Reisejournal{{end}}
+
+{{define "content"}}
+<main class="container">
+  <div class="page-header">
+    <h1>REISEJOURNAL</h1>
+    <a href="/logout">[ Ausloggen ]</a>
+  </div>
+  <form method="get" action="/days/redirect">
+    <fieldset role="group">
+      <input type="date" name="date" id="nav-date" required>
+      <button type="submit">Tag öffnen</button>
+    </fieldset>
+  </form>
+
+  <h2>Reisetage</h2>
+  <figure>
+  <table>
+    <thead><tr><th>Datum</th><th>Punkte</th><th>Von</th><th>Bis</th></tr></thead>
+    <tbody>
+    {{range .Days}}
+    <tr>
+      <td><a href="/days/{{.Date}}">{{.Date}}</a></td>
+      <td>{{.Count}}</td>
+      <td><small>{{if .FirstTS}}{{.FirstTS.Format "15:04"}}{{end}}</small></td>
+      <td><small>{{if .LastTS}}{{.LastTS.Format "15:04"}}{{end}}</small></td>
+    </tr>
+    {{else}}
+    <tr><td colspan="4"><small>// Keine Daten vorhanden</small></td></tr>
+    {{end}}
+    </tbody>
+  </table>
+  </figure>
+</main>
+{{end}}
+
+{{define "scripts"}}
+<script>
+  var d = document.getElementById('nav-date');
+  if (d && !d.value) {
+    d.value = new Date().toISOString().slice(0, 10);
+  }
+</script>
+{{end}}
+
+{{template "base" .}}