fix: Date/Message-ID/From-name on ticket emails, richer body (1.6.1-aio.9)

A tester's ticket email landed in spam. A mail-tester run against demo
scored 8.3/10 with SPF, DKIM and DMARC all passing through the VPS relay,
so the deductions were all in the message: MISSING_DATE (1.4),
HTML_IMAGE_ONLY_04 (0.3), MISSING_MID (0.1) — and Gmail/Outlook weigh a
missing Date/Message-ID as "machine-generated" far more than that.

- `build_ticket_email` sets Date, a Message-ID under the sender domain,
  and a From display name from `lnbits_site_title`.
- The body now carries the event name, dates, location, name on ticket,
  ticket id and the door instruction, so the HTML part is no longer a
  QR with a handful of words.

Upstream candidate: lnbits core `send_email` has the same omissions.

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

config.json

@@ -1,6 +1,6 @@
 {
   "id": "events",
-  "version": "1.6.1-aio.8",
+  "version": "1.6.1-aio.9",
   "name": "Events",
   "repo": "https://git.atitlan.io/aiolabs/events",
   "short_description": "Sell and register event tickets",