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>

docker/init/00-databases.sql

@@ -0,0 +1,10 @@
+-- Local dev only. Creates the three databases + a dev user with full access.
+CREATE DATABASE IF NOT EXISTS nachklang_calendar CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
+CREATE DATABASE IF NOT EXISTS nachklang_feedback CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
+CREATE DATABASE IF NOT EXISTS nachklang_tickets CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
+
+CREATE USER IF NOT EXISTS 'nachklang'@'%' IDENTIFIED BY 'devpassword';
+GRANT ALL PRIVILEGES ON nachklang_calendar.* TO 'nachklang'@'%';
+GRANT ALL PRIVILEGES ON nachklang_feedback.* TO 'nachklang'@'%';
+GRANT ALL PRIVILEGES ON nachklang_tickets.* TO 'nachklang'@'%';
+FLUSH PRIVILEGES;