Implement event domain model and application service

Add Event entity, CreateEventCommand, ports (CreateEventUseCase,
EventRepository), and EventService with Clock injection for
deterministic testing. Expiry date must be in the future.

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

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

@@ -0,0 +1,109 @@
+package de.fete.domain.model;
+
+import java.time.LocalDate;
+import java.time.OffsetDateTime;
+import java.util.UUID;
+
+/** Domain entity representing an event. */
+public class Event {
+
+  private Long id;
+  private UUID eventToken;
+  private UUID organizerToken;
+  private String title;
+  private String description;
+  private OffsetDateTime dateTime;
+  private String location;
+  private LocalDate expiryDate;
+  private OffsetDateTime createdAt;
+
+  /** Returns the internal database ID. */
+  public Long getId() {
+    return id;
+  }
+
+  /** Sets the internal database ID. */
+  public void setId(Long id) {
+    this.id = id;
+  }
+
+  /** Returns the public event token (UUID). */
+  public UUID getEventToken() {
+    return eventToken;
+  }
+
+  /** Sets the public event token. */
+  public void setEventToken(UUID eventToken) {
+    this.eventToken = eventToken;
+  }
+
+  /** Returns the secret organizer token (UUID). */
+  public UUID getOrganizerToken() {
+    return organizerToken;
+  }
+
+  /** Sets the secret organizer token. */
+  public void setOrganizerToken(UUID organizerToken) {
+    this.organizerToken = organizerToken;
+  }
+
+  /** Returns the event title. */
+  public String getTitle() {
+    return title;
+  }
+
+  /** Sets the event title. */
+  public void setTitle(String title) {
+    this.title = title;
+  }
+
+  /** Returns the event description. */
+  public String getDescription() {
+    return description;
+  }
+
+  /** Sets the event description. */
+  public void setDescription(String description) {
+    this.description = description;
+  }
+
+  /** Returns the event date and time with UTC offset. */
+  public OffsetDateTime getDateTime() {
+    return dateTime;
+  }
+
+  /** Sets the event date and time. */
+  public void setDateTime(OffsetDateTime dateTime) {
+    this.dateTime = dateTime;
+  }
+
+  /** Returns the event location. */
+  public String getLocation() {
+    return location;
+  }
+
+  /** Sets the event location. */
+  public void setLocation(String location) {
+    this.location = location;
+  }
+
+  /** Returns the expiry date after which event data is deleted. */
+  public LocalDate getExpiryDate() {
+    return expiryDate;
+  }
+
+  /** Sets the expiry date. */
+  public void setExpiryDate(LocalDate expiryDate) {
+    this.expiryDate = expiryDate;
+  }
+
+  /** Returns the creation timestamp. */
+  public OffsetDateTime getCreatedAt() {
+    return createdAt;
+  }
+
+  /** Sets the creation timestamp. */
+  public void setCreatedAt(OffsetDateTime createdAt) {
+    this.createdAt = createdAt;
+  }
+}