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/main/java/de/fete/adapter/in/web/GlobalExceptionHandler.java

@@ -1,5 +1,6 @@
 package de.fete.adapter.in.web;
 
+import de.fete.application.service.EventExpiredException;
 import de.fete.application.service.EventNotFoundException;
 import de.fete.application.service.ExpiryDateInPastException;
 import de.fete.application.service.InvalidTimezoneException;
@@ -59,6 +60,19 @@ public class GlobalExceptionHandler extends ResponseEntityExceptionHandler {
         .body(problemDetail);
   }
 
+  /** Handles RSVP on expired event. */
+  @ExceptionHandler(EventExpiredException.class)
+  public ResponseEntity<ProblemDetail> handleEventExpired(
+      EventExpiredException ex) {
+    ProblemDetail problemDetail = ProblemDetail.forStatusAndDetail(
+        HttpStatus.CONFLICT, ex.getMessage());
+    problemDetail.setTitle("Event Expired");
+    problemDetail.setType(URI.create("urn:problem-type:event-expired"));
+    return ResponseEntity.status(HttpStatus.CONFLICT)
+        .contentType(MediaType.APPLICATION_PROBLEM_JSON)
+        .body(problemDetail);
+  }
+
   /** Handles event not found. */
   @ExceptionHandler(EventNotFoundException.class)
   public ResponseEntity<ProblemDetail> handleEventNotFound(

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

@@ -8,6 +8,8 @@ import de.fete.domain.port.in.CountAttendeesByEventUseCase;
 import de.fete.domain.port.in.CreateRsvpUseCase;
 import de.fete.domain.port.out.EventRepository;
 import de.fete.domain.port.out.RsvpRepository;
+import java.time.Clock;
+import java.time.LocalDate;
 import org.springframework.stereotype.Service;
 
 /** Application service implementing RSVP creation. */
@@ -16,13 +18,16 @@ public class RsvpService implements CreateRsvpUseCase, CountAttendeesByEventUseC
 
   private final EventRepository eventRepository;
   private final RsvpRepository rsvpRepository;
+  private final Clock clock;
 
   /** Creates a new RsvpService. */
   public RsvpService(
       EventRepository eventRepository,
-      RsvpRepository rsvpRepository) {
+      RsvpRepository rsvpRepository,
+      Clock clock) {
     this.eventRepository = eventRepository;
     this.rsvpRepository = rsvpRepository;
+    this.clock = clock;
   }
 
   @Override
@@ -30,6 +35,10 @@ public class RsvpService implements CreateRsvpUseCase, CountAttendeesByEventUseC
     Event event = eventRepository.findByEventToken(eventToken)
         .orElseThrow(() -> new EventNotFoundException(eventToken.value()));
 
+    if (!event.getExpiryDate().isAfter(LocalDate.now(clock))) {
+      throw new EventExpiredException(eventToken.value());
+    }
+
     var rsvp = new Rsvp();
     rsvp.setRsvpToken(RsvpToken.generate());
     rsvp.setEventId(event.getId());