feat: Start/Ende fuer Fixposten in der GUI + Datumsvalidierung

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

finance/app/templates/planning.html

@@ -7,7 +7,7 @@
   <h2>Wiederkehrende Posten</h2>
   <table>
     <thead>
-      <tr><th>Name</th><th>Betrag</th><th>Rhythmus</th><th>Fälligkeitstag</th><th>Kategorie</th><th></th></tr>
+      <tr><th>Name</th><th>Betrag</th><th>Rhythmus</th><th>Fälligkeitstag</th><th>Start</th><th>Ende</th><th>Kategorie</th><th></th></tr>
     </thead>
     <tbody>
       {% for r in recurring %}
@@ -16,6 +16,8 @@
         <td class="{{ 'neg' if r.amount < 0 else '' }}">{{ r.amount|eur }} €</td>
         <td>{{ r.rhythm|de_label }}</td>
         <td>{{ r.due_day }}</td>
+        <td>{{ r.start_date.strftime('%d.%m.%Y') if r.start_date else '–' }}</td>
+        <td>{{ r.end_date.strftime('%d.%m.%Y') if r.end_date else '–' }}</td>
         <td>{{ category_names.get(r.category_id, '–') }}</td>
         <td>
           <form class="inline-form" hx-delete="/api/recurring/{{ r.id }}" hx-swap="none"
@@ -26,7 +28,7 @@
         </td>
       </tr>
       {% else %}
-      <tr><td colspan="6">Noch keine wiederkehrenden Posten.</td></tr>
+      <tr><td colspan="8">Noch keine wiederkehrenden Posten.</td></tr>
       {% endfor %}
     </tbody>
   </table>
@@ -43,6 +45,8 @@
         </select>
       </label>
       <label>Fälligkeitstag <input type="number" name="due_day" data-type="int" min="1" max="31" required></label>
+      <label>Start (optional) <input type="date" name="start_date"></label>
+      <label>Ende (optional) <input type="date" name="end_date"></label>
       <label>Kategorie
         <select name="category_id" data-type="int">
           <option value="">–</option>