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

README.md

@@ -67,7 +67,10 @@ Events includes a shareable ticket scanner, which can be used to register attend
   instance logo, name on ticket, ticket id) also served at
   `GET /events/api/v1/ticket-card/{ticket_id}`; the bare QR stays at
   `GET /events/api/v1/qr/{ticket_id}`. Headers carry Date, Message-ID and a
-  From display name (site title). `POST /events/api/v1/tickets/{ticket_id}/resend-email`
+  From display name — `extra.organizer_name` gives "Organizer via <site title>",
+  otherwise the site title. Replies go to `extra.reply_to_email`, falling back
+  to the event owner's account email (no Reply-To when neither exists). The
+  From _address_ is always the instance mailbox, which is what DKIM signs. `POST /events/api/v1/tickets/{ticket_id}/resend-email`
   returns a `TicketResendResult` with per-channel outcome.
 
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