Block RSVPs on expired events with 409 Conflict and inject Clock into RsvpService

Adds expiry check to RsvpService using an injected Clock for testability,
handles EventExpiredException in GlobalExceptionHandler as 409 Conflict,
and adds unit + integration tests using relative dates from a fixed clock.

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

backend/src/test/java/de/fete/adapter/in/web/EventControllerIntegrationTest.java

@@ -349,6 +349,22 @@ class EventControllerIntegrationTest {
         .andExpect(jsonPath("$.type").value("urn:problem-type:event-not-found"));
   }
 
+  @Test
+  void createRsvpForExpiredEventReturns409() throws Exception {
+    EventJpaEntity event = seedEvent(
+        "Expired Party", null, "Europe/Berlin",
+        null, LocalDate.now().minusDays(1));
+
+    var request = new CreateRsvpRequest().name("Late Guest");
+
+    mockMvc.perform(post("/api/events/" + event.getEventToken() + "/rsvps")
+            .contentType(MediaType.APPLICATION_JSON)
+            .content(objectMapper.writeValueAsString(request)))
+        .andExpect(status().isConflict())
+        .andExpect(content().contentTypeCompatibleWith("application/problem+json"))
+        .andExpect(jsonPath("$.type").value("urn:problem-type:event-expired"));
+  }
+
   private EventJpaEntity seedEvent(
       String title, String description, String timezone,
       String location, LocalDate expiryDate) {

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

@@ -12,6 +12,8 @@ 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;
@@ -28,6 +30,9 @@ import org.mockito.junit.jupiter.MockitoExtension;
 class RsvpServiceTest {
 
   private static final ZoneId ZONE = ZoneId.of("Europe/Berlin");
+  private static final Instant NOW = Instant.parse("2026-03-08T12:00:00Z");
+  private static final Clock FIXED_CLOCK = Clock.fixed(NOW, ZONE);
+  private static final LocalDate TODAY = LocalDate.ofInstant(NOW, ZONE);
 
   @Mock
   private EventRepository eventRepository;
@@ -39,7 +44,7 @@ class RsvpServiceTest {
 
   @BeforeEach
   void setUp() {
-    rsvpService = new RsvpService(eventRepository, rsvpRepository);
+    rsvpService = new RsvpService(eventRepository, rsvpRepository, FIXED_CLOCK);
   }
 
   @Test
@@ -95,6 +100,28 @@ class RsvpServiceTest {
     assertThat(result.getName()).isEqualTo("Max");
   }
 
+  @Test
+  void createRsvpThrowsWhenEventExpired() {
+    var event = buildActiveEvent();
+    event.setExpiryDate(TODAY.minusDays(1));
+    EventToken token = event.getEventToken();
+    when(eventRepository.findByEventToken(token)).thenReturn(Optional.of(event));
+
+    assertThatThrownBy(() -> rsvpService.createRsvp(token, "Late Guest"))
+        .isInstanceOf(EventExpiredException.class);
+  }
+
+  @Test
+  void createRsvpThrowsWhenEventExpiresToday() {
+    var event = buildActiveEvent();
+    event.setExpiryDate(TODAY);
+    EventToken token = event.getEventToken();
+    when(eventRepository.findByEventToken(token)).thenReturn(Optional.of(event));
+
+    assertThatThrownBy(() -> rsvpService.createRsvp(token, "Late Guest"))
+        .isInstanceOf(EventExpiredException.class);
+  }
+
   private Event buildActiveEvent() {
     var event = new Event();
     event.setId(1L);
@@ -103,7 +130,7 @@ class RsvpServiceTest {
     event.setTitle("Test Event");
     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.setExpiryDate(TODAY.plusDays(30));
     event.setCreatedAt(OffsetDateTime.now());
     return event;
   }