Add Tickets domain for the voucher-based ticket shop

New domain mirroring the Feedback module's conventions: single-use voucher
codes (wildcard + personalized), redemption with race-safe capacity and
deadline enforcement, admin CRUD for vouchers/redemptions/event settings
with an audit trail, and reuse of the existing Calendar session auth.

Backend pieces:
- Tickets domain (public redeem flow, admin vouchers/redemptions/events)
- Calendar events.service.ts: add getEventById
- Mailer: attachment/HTML support, fix a shared-mutable-state race
- Per-event capacity/deadline/address settings, with an "address
  required" option on top of "address collected"
- Email format validation on redeem and admin voucher/redemption input
- Docker Compose dev stack (MariaDB + seed data) for local testing

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

sql/tickets/002_add_require_address.sql

@@ -0,0 +1,7 @@
+-- Nachklang e.V. Tickets module — adds a per-event "address required" flag,
+-- distinct from collect_address (which only controls whether the field is
+-- shown/collected at all). Apply manually against TICKETS_DB, after
+-- 001_init.sql:
+--   mysql -h <DB_HOST> -u <DB_USER> -p <TICKETS_DB> < 002_add_require_address.sql
+ALTER TABLE event_ticket_settings
+    ADD COLUMN require_address TINYINT(1) NOT NULL DEFAULT 0 AFTER collect_address;