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-compose.dev.yml

@@ -0,0 +1,28 @@
+# Local dev only — not used in production/deployment. Spins up a MariaDB
+# instance with the calendar (reconstructed dev schema, see docker/init's
+# disclaimer), feedback, and tickets databases pre-seeded.
+#
+# Usage:
+#   docker compose -f docker-compose.dev.yml up -d
+#
+# Then point .env at:
+#   DB_HOST=127.0.0.1
+#   DB_USER=nachklang
+#   DB_PASSWORD=devpassword
+#   CALENDAR_DB=nachklang_calendar
+#   FEEDBACK_DB=nachklang_feedback
+#   TICKETS_DB=nachklang_tickets
+services:
+  mariadb:
+    image: mariadb:11
+    environment:
+      MARIADB_ROOT_PASSWORD: rootdevpassword
+    ports:
+      - "3306:3306"
+    volumes:
+      - nachklang_dev_db:/var/lib/mysql
+      - ./sql:/migrations:ro
+      - ./docker/init:/docker-entrypoint-initdb.d:ro
+
+volumes:
+  nachklang_dev_db: