Make customer upload-confirmation email opt-out via settings

New smtp_notify_customer toggle in the SMTP section. Defaults to true
to preserve existing behavior. When unchecked, customers no longer
receive the upload summary even if they have an email on file; staff
and (optionally) admins keep getting their notifications.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/admin/index.html

@@ -379,6 +379,12 @@
             <div><label>Absender (From)</label><input name="smtp_from" placeholder="noreply@firma.de" /></div>
           </div>
           <div class="field" style="margin-top:.75rem">
+            <label style="display:flex; align-items:center; gap:.4rem">
+              <input type="checkbox" name="smtp_notify_customer" style="width:auto" />
+              Kunden nach Upload eine Bestätigungsmail schicken (nur wenn E-Mail beim Kunden hinterlegt)
+            </label>
+          </div>
+          <div class="field" style="margin-top:.5rem">
             <label style="display:flex; align-items:center; gap:.4rem">
               <input type="checkbox" name="smtp_notify_admin" style="width:auto" />
               Admins bei jedem Upload benachrichtigen (alle Admins mit E-Mail-Adresse)
@@ -765,6 +771,7 @@ async function loadSettings() {
     ? 'gesetzt — leer = unverändert lassen'
     : 'nicht gesetzt';
   sf.smtp_notify_admin.checked = !!s.smtp_notify_admin;
+  sf.smtp_notify_customer.checked = s.smtp_notify_customer !== false;
 }
 function setSlider(id, val) {
   document.getElementById(id).value = val;
@@ -819,6 +826,7 @@ document.getElementById('smtpForm').addEventListener('submit', async (e) => {
     smtp_user: fd.get('smtp_user') || '',
     smtp_from: fd.get('smtp_from') || '',
     smtp_notify_admin: !!fd.get('smtp_notify_admin'),
+    smtp_notify_customer: !!fd.get('smtp_notify_customer'),
   };
   const pw = fd.get('smtp_pass');
   if (pw) body.smtp_pass = pw;