Extract CountAttendeesByEventUseCase to decouple controller from repository

The EventController was directly accessing RsvpRepository (an outbound port)
to count attendees, bypassing the application layer. Introduce a dedicated
inbound port and implement it in RsvpService. Remove the now-unused Clock
dependency from RsvpService.

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

backend/src/test/java/de/fete/application/service/RsvpServiceTest.java

@@ -12,8 +12,6 @@ import de.fete.domain.model.OrganizerToken;
 import de.fete.domain.model.Rsvp;
 import de.fete.domain.port.out.EventRepository;
 import de.fete.domain.port.out.RsvpRepository;
-import java.time.Clock;
-import java.time.Instant;
 import java.time.LocalDate;
 import java.time.OffsetDateTime;
 import java.time.ZoneId;
@@ -30,9 +28,6 @@ import org.mockito.junit.jupiter.MockitoExtension;
 class RsvpServiceTest {
 
   private static final ZoneId ZONE = ZoneId.of("Europe/Berlin");
-  private static final Instant FIXED_INSTANT =
-      LocalDate.of(2026, 3, 5).atStartOfDay(ZONE).toInstant();
-  private static final Clock FIXED_CLOCK = Clock.fixed(FIXED_INSTANT, ZONE);
 
   @Mock
   private EventRepository eventRepository;
@@ -44,7 +39,7 @@ class RsvpServiceTest {
 
   @BeforeEach
   void setUp() {
-    rsvpService = new RsvpService(eventRepository, rsvpRepository, FIXED_CLOCK);
+    rsvpService = new RsvpService(eventRepository, rsvpRepository);
   }
 
   @Test
@@ -109,7 +104,7 @@ class RsvpServiceTest {
     event.setDateTime(OffsetDateTime.of(2026, 6, 15, 20, 0, 0, 0, ZoneOffset.ofHours(2)));
     event.setTimezone(ZONE);
     event.setExpiryDate(LocalDate.of(2026, 7, 15));
-    event.setCreatedAt(OffsetDateTime.now(FIXED_CLOCK));
+    event.setCreatedAt(OffsetDateTime.now());
     return event;
   }
 }