Fix monthly PDF export to include only the selected month

The month dropdown carried only the month (e.g. "07") while the year was
a separate select; the export required month AND year, so a month-only
selection fell through to exporting every application. Encode the year in
the month option value ("YYYY-MM"), parse it client-side to always send
the exact month+year, and harden /api/export so a month can never fall
through to "export all".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

views/index.ejs

@@ -568,11 +568,11 @@
                             class="w-full px-3 py-2 border border-gray-300 dark:border-gray-600 rounded-md bg-white dark:bg-gray-700 text-gray-800 dark:text-white">
                             <option value="">-- Alle Monate --</option>
                             <% availableMonths.forEach(m => { %>
-                                <option value="<%= m.month %>"><%= m.month %> - <%= m.year %></option>
+                                <option value="<%= m.year %>-<%= m.month %>"><%= m.month %> - <%= m.year %></option>
                             <% }); %>
                         </select>
                     </div>
-                    
+
                     <div>
                         <label for="pdfYear" class="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-2">
                             Jahr