Implement GET /events/{token} backend with timezone support

Domain: add timezone field to Event and CreateEventCommand.
Ports: new GetEventUseCase inbound port.
Service: implement getByEventToken, validate IANA timezone on create.
Controller: map to GetEventResponse, compute expired flag via Clock.
Persistence: timezone column in JPA entity and mapping.
Tests: integration tests use DTOs + ObjectMapper instead of inline JSON,
GET tests seed DB directly via JpaRepository for isolation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/main/java/de/fete/domain/model/Event.java

@@ -2,6 +2,7 @@ package de.fete.domain.model;
 
 import java.time.LocalDate;
 import java.time.OffsetDateTime;
+import java.time.ZoneId;
 import java.util.UUID;
 
 /** Domain entity representing an event. */
@@ -13,6 +14,7 @@ public class Event {
   private String title;
   private String description;
   private OffsetDateTime dateTime;
+  private ZoneId timezone;
   private String location;
   private LocalDate expiryDate;
   private OffsetDateTime createdAt;
@@ -77,6 +79,16 @@ public class Event {
     this.dateTime = dateTime;
   }
 
+  /** Returns the IANA timezone. */
+  public ZoneId getTimezone() {
+    return timezone;
+  }
+
+  /** Sets the IANA timezone. */
+  public void setTimezone(ZoneId timezone) {
+    this.timezone = timezone;
+  }
+
   /** Returns the event location. */
   public String getLocation() {
     return location;