Add cancel RSVP feature (backend DELETE endpoint + frontend UI)

Allows guests to cancel their RSVP via a DELETE endpoint using their
guestToken. Frontend shows cancel button in RsvpBar and clears local
storage on success. Includes unit tests, integration tests, and E2E spec.

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

backend/src/main/java/de/fete/adapter/in/web/EventController.java

@@ -15,6 +15,8 @@ import de.fete.domain.model.Event;
 import de.fete.domain.model.EventToken;
 import de.fete.domain.model.OrganizerToken;
 import de.fete.domain.model.Rsvp;
+import de.fete.domain.model.RsvpToken;
+import de.fete.domain.port.in.CancelRsvpUseCase;
 import de.fete.domain.port.in.CountAttendeesByEventUseCase;
 import de.fete.domain.port.in.CreateEventUseCase;
 import de.fete.domain.port.in.CreateRsvpUseCase;
@@ -35,6 +37,7 @@ public class EventController implements EventsApi {
   private final CreateEventUseCase createEventUseCase;
   private final GetEventUseCase getEventUseCase;
   private final CreateRsvpUseCase createRsvpUseCase;
+  private final CancelRsvpUseCase cancelRsvpUseCase;
   private final CountAttendeesByEventUseCase countAttendeesByEventUseCase;
   private final GetAttendeesUseCase getAttendeesUseCase;
 
@@ -43,11 +46,13 @@ public class EventController implements EventsApi {
       CreateEventUseCase createEventUseCase,
       GetEventUseCase getEventUseCase,
       CreateRsvpUseCase createRsvpUseCase,
+      CancelRsvpUseCase cancelRsvpUseCase,
       CountAttendeesByEventUseCase countAttendeesByEventUseCase,
       GetAttendeesUseCase getAttendeesUseCase) {
     this.createEventUseCase = createEventUseCase;
     this.getEventUseCase = getEventUseCase;
     this.createRsvpUseCase = createRsvpUseCase;
+    this.cancelRsvpUseCase = cancelRsvpUseCase;
     this.countAttendeesByEventUseCase = countAttendeesByEventUseCase;
     this.getAttendeesUseCase = getAttendeesUseCase;
   }
@@ -128,6 +133,12 @@ public class EventController implements EventsApi {
     return ResponseEntity.status(HttpStatus.CREATED).body(response);
   }
 
+  @Override
+  public ResponseEntity<Void> cancelRsvp(UUID token, UUID rsvpToken) {
+    cancelRsvpUseCase.cancelRsvp(new EventToken(token), new RsvpToken(rsvpToken));
+    return ResponseEntity.noContent().build();
+  }
+
   private static ZoneId parseTimezone(String timezone) {
     try {
       return ZoneId.of(timezone);

backend/src/main/java/de/fete/adapter/out/persistence/RsvpJpaRepository.java

@@ -14,4 +14,7 @@ public interface RsvpJpaRepository extends JpaRepository<RsvpJpaEntity, Long> {
 
   /** Finds all RSVPs for the given event, ordered by ID ascending. */
   java.util.List<RsvpJpaEntity> findAllByEventIdOrderByIdAsc(Long eventId);
+
+  /** Deletes an RSVP by event ID and RSVP token. Returns count of deleted rows. */
+  long deleteByEventIdAndRsvpToken(Long eventId, UUID rsvpToken);
 }

backend/src/main/java/de/fete/adapter/out/persistence/RsvpPersistenceAdapter.java

@@ -36,6 +36,11 @@ public class RsvpPersistenceAdapter implements RsvpRepository {
         .toList();
   }
 
+  @Override
+  public boolean deleteByEventIdAndRsvpToken(Long eventId, RsvpToken rsvpToken) {
+    return jpaRepository.deleteByEventIdAndRsvpToken(eventId, rsvpToken.value()) > 0;
+  }
+
   private RsvpJpaEntity toEntity(Rsvp rsvp) {
     var entity = new RsvpJpaEntity();
     entity.setId(rsvp.getId());

backend/src/main/java/de/fete/application/service/RsvpService.java

@@ -5,11 +5,13 @@ import de.fete.domain.model.EventToken;
 import de.fete.domain.model.OrganizerToken;
 import de.fete.domain.model.Rsvp;
 import de.fete.domain.model.RsvpToken;
+import de.fete.domain.port.in.CancelRsvpUseCase;
 import de.fete.domain.port.in.CountAttendeesByEventUseCase;
 import de.fete.domain.port.in.CreateRsvpUseCase;
 import de.fete.domain.port.in.GetAttendeesUseCase;
 import de.fete.domain.port.out.EventRepository;
 import de.fete.domain.port.out.RsvpRepository;
+import jakarta.transaction.Transactional;
 import java.time.Clock;
 import java.time.LocalDate;
 import java.util.List;
@@ -18,7 +20,8 @@ import org.springframework.stereotype.Service;
 /** Application service implementing RSVP operations. */
 @Service
 public class RsvpService
-    implements CreateRsvpUseCase, CountAttendeesByEventUseCase, GetAttendeesUseCase {
+    implements CreateRsvpUseCase, CancelRsvpUseCase, CountAttendeesByEventUseCase,
+    GetAttendeesUseCase {
 
   private final EventRepository eventRepository;
   private final RsvpRepository rsvpRepository;
@@ -51,6 +54,14 @@ public class RsvpService
     return rsvpRepository.save(rsvp);
   }
 
+  @Override
+  @Transactional
+  public void cancelRsvp(EventToken eventToken, RsvpToken rsvpToken) {
+    eventRepository.findByEventToken(eventToken)
+        .ifPresent(event ->
+            rsvpRepository.deleteByEventIdAndRsvpToken(event.getId(), rsvpToken));
+  }
+
   @Override
   public long countByEvent(EventToken eventToken) {
     Event event = eventRepository.findByEventToken(eventToken)

backend/src/main/java/de/fete/domain/port/in/CancelRsvpUseCase.java

@@ -0,0 +1,11 @@
+package de.fete.domain.port.in;
+
+import de.fete.domain.model.EventToken;
+import de.fete.domain.model.RsvpToken;
+
+/** Inbound port for cancelling an RSVP. */
+public interface CancelRsvpUseCase {
+
+  /** Cancels the RSVP identified by the given tokens. Idempotent — no error if not found. */
+  void cancelRsvp(EventToken eventToken, RsvpToken rsvpToken);
+}

backend/src/main/java/de/fete/domain/port/out/RsvpRepository.java

@@ -1,6 +1,7 @@
 package de.fete.domain.port.out;
 
 import de.fete.domain.model.Rsvp;
+import de.fete.domain.model.RsvpToken;
 import java.util.List;
 
 /** Outbound port for persisting and querying RSVPs. */
@@ -14,4 +15,7 @@ public interface RsvpRepository {
 
   /** Finds all RSVPs for the given event, ordered by ID ascending. */
   List<Rsvp> findByEventId(Long eventId);
+
+  /** Deletes an RSVP by event ID and RSVP token. Returns true if a record was deleted. */
+  boolean deleteByEventIdAndRsvpToken(Long eventId, RsvpToken rsvpToken);
 }

backend/src/main/resources/openapi/api.yaml

@@ -37,6 +37,38 @@ paths:
               schema:
                 $ref: "#/components/schemas/ValidationProblemDetail"
 
+  /events/{token}/rsvps/{rsvpToken}:
+    delete:
+      operationId: cancelRsvp
+      summary: Cancel RSVP
+      description: |
+        Permanently deletes an RSVP identified by the RSVP token.
+        Idempotent: returns 204 whether the RSVP existed or not.
+      tags:
+        - events
+      parameters:
+        - name: token
+          in: path
+          required: true
+          schema:
+            type: string
+            format: uuid
+          description: Event token (UUID)
+        - name: rsvpToken
+          in: path
+          required: true
+          schema:
+            type: string
+            format: uuid
+          description: RSVP token (UUID) identifying the attendance to cancel
+      responses:
+        "204":
+          description: >
+            RSVP successfully cancelled (or was already cancelled).
+            No response body.
+        "500":
+          description: Internal server error
+
   /events/{token}/rsvps:
     post:
       operationId: createRsvp