feat: per-event organizer name + reply-to on ticket emails

Each organizer is different, so the sender identity of ticket emails is
now per event rather than per instance:

- `extra.organizer_name` → From display name "Organizer via <site title>"
  (site title alone when unset). The From address stays the instance
  mailbox — that is what DKIM signs — so this costs nothing in mail auth.
- `extra.reply_to_email` → Reply-To; blank falls back to the email on the
  LNbits account that owns the event wallet; no header when neither
  exists. When a reply-to exists the body says "Questions? Reply to this
  email and it reaches <organizer>", and "Organizer: <name>" is listed
  with the ticket details.

Both fields sit next to the existing per-event subject/body in the admin
dialog. The organizer's own wording (in whatever language) remains
`notification_body`.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EYwoAkZZmXMMmaBp4WGUBo

static/js/index.js

@@ -206,6 +206,8 @@ window.PageEvents = {
           extra: {
             payment_methods: ['lightning'],
             promo_codes: [],
+            organizer_name: '',
+            reply_to_email: '',
             notification_subject: '',
             notification_body: ''
           }
@@ -582,6 +584,8 @@ window.PageEvents = {
             email_notifications: false,
             nostr_notifications: false,
             promo_codes: [],
+            organizer_name: '',
+            reply_to_email: '',
             notification_subject: '',
             notification_body: ''
           }
@@ -602,6 +606,8 @@ window.PageEvents = {
           email_notifications: false,
           nostr_notifications: false,
           promo_codes: [],
+          organizer_name: '',
+          reply_to_email: '',
           notification_subject: '',
           notification_body: ''
         }