Let a form send from its own address in 1.15.1

Umut asked where a contact form's mail is sent from: it was the recipient
address, `From` equal to `To`. That breaks as soon as the recipient is a
mailbox at another provider, because most hosts only deliver mail that
claims an address of their own domain, and the provider's SPF/DMARC then
treats it as forged.

Each form now has an optional `sender` setting, validated like the
recipient; empty keeps the old behaviour, so stored forms need no
migration. Delivery uses `sender ?: recipient`; the reply-to field is
untouched. The administration shows the field under the recipient with a
note on why it matters. The pilot launch config moves to port 8091 because
8087 was held by another session.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

templates/admin/forms.html.twig

@@ -96,6 +96,11 @@
                         <label for="recipient-{{ f }}">Empfänger</label>
                         <input id="recipient-{{ f }}" name="recipient" type="email" required value="{{ settings.recipient|default('') }}" placeholder="info@beispiel.de">
                     </div>
+                    <div>
+                        <label for="sender-{{ f }}">Absender</label>
+                        <input id="sender-{{ f }}" name="sender" type="email" value="{{ settings.sender|default('') }}" placeholder="formular@beispiel.de">
+                        <p class="field-help">Leer: die Empfängeradresse. Viele Anbieter stellen nur zu, was von einer Adresse der eigenen Domain kommt.</p>
+                    </div>
                     <div>
                         <label for="subject-{{ f }}">Betreff der E-Mail</label>
                         <input id="subject-{{ f }}" name="subject" maxlength="160" value="{{ settings.subject|default('') }}" placeholder="Neue Formularnachricht">