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/test/java/de/fete/application/service/EventServiceTest.java

@@ -9,6 +9,7 @@ import static org.mockito.Mockito.when;
 
 import de.fete.domain.model.CreateEventCommand;
 import de.fete.domain.model.Event;
+import de.fete.domain.model.EventToken;
 import de.fete.domain.port.out.EventRepository;
 import java.time.Clock;
 import java.time.Instant;
@@ -17,7 +18,6 @@ import java.time.OffsetDateTime;
 import java.time.ZoneId;
 import java.time.ZoneOffset;
 import java.util.Optional;
-import java.util.UUID;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
@@ -130,7 +130,7 @@ class EventServiceTest {
 
   @Test
   void getByEventTokenReturnsEvent() {
-    UUID token = UUID.randomUUID();
+    EventToken token = EventToken.generate();
     var event = new Event();
     event.setEventToken(token);
     event.setTitle("Found Event");
@@ -145,7 +145,7 @@ class EventServiceTest {
 
   @Test
   void getByEventTokenReturnsEmptyForUnknownToken() {
-    UUID token = UUID.randomUUID();
+    EventToken token = EventToken.generate();
     when(eventRepository.findByEventToken(token))
         .thenReturn(Optional.empty());