Add RSVP creation endpoint with typed tokens and attendee count

Introduce typed token value objects (EventToken, OrganizerToken,
RsvpToken) and refactor all existing Event code to use them.

Add POST /events/{token}/rsvps endpoint that persists an RSVP and
returns an rsvpToken. Populate attendeeCount in GET /events/{token}
from a real count query instead of hardcoded 0.

Includes: OpenAPI spec, Liquibase migration (rsvps table with
ON DELETE CASCADE), domain model, hexagonal ports/adapters,
service layer, and full test coverage (unit + integration).

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

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

@@ -3,14 +3,13 @@ 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. */
 public class Event {
 
   private Long id;
-  private UUID eventToken;
-  private UUID organizerToken;
+  private EventToken eventToken;
+  private OrganizerToken organizerToken;
   private String title;
   private String description;
   private OffsetDateTime dateTime;
@@ -29,23 +28,23 @@ public class Event {
     this.id = id;
   }
 
-  /** Returns the public event token (UUID). */
-  public UUID getEventToken() {
+  /** Returns the public event token. */
+  public EventToken getEventToken() {
     return eventToken;
   }
 
   /** Sets the public event token. */
-  public void setEventToken(UUID eventToken) {
+  public void setEventToken(EventToken eventToken) {
     this.eventToken = eventToken;
   }
 
-  /** Returns the secret organizer token (UUID). */
-  public UUID getOrganizerToken() {
+  /** Returns the secret organizer token. */
+  public OrganizerToken getOrganizerToken() {
     return organizerToken;
   }
 
   /** Sets the secret organizer token. */
-  public void setOrganizerToken(UUID organizerToken) {
+  public void setOrganizerToken(OrganizerToken organizerToken) {
     this.organizerToken = organizerToken;
   }