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/login.html

@@ -0,0 +1,21 @@
+{{define "title"}}Login — Reisejournal{{end}}
+
+{{define "content"}}
+<main class="container">
+<article class="login-box">
+  <h1>REISEJOURNAL</h1>
+  {{if .Error}}<p class="err">// {{.Error}}</p>{{end}}
+  <form method="post" action="/login">
+    <label>Benutzername
+      <input name="username" autocomplete="username" value="{{.Username}}">
+    </label>
+    <label>Passwort
+      <input type="password" name="password" autocomplete="current-password">
+    </label>
+    <button type="submit">Einloggen</button>
+  </form>
+</article>
+</main>
+{{end}}
+
+{{template "base" .}}