✨(mail) include expiration information in recording notification emails

Add validity duration (number of days valid) to email
notifications for recordings. Informs users about their recording's lifespan,
providing important context about content availability.

src/mail/mjml/screen_recording.mjml

@@ -23,6 +23,11 @@
             {% blocktrans %}
             Your recording of "{{room_name}}" on {{recording_date}} at {{recording_time}} is now ready to download.
             {% endblocktrans %}
+            {% if recording_expiration_days %}
+            {% blocktrans with days=recording_expiration_days %}
+            The recording will expire in {{days}} days.
+            {% endblocktrans %}
+            {% endif %}
           </mj-text>
           <mj-text>
             <p>{% trans "To keep this recording permanently:" %}</p>