Validate expiryDate is strictly after eventDate and harden rejection tests

Adds ExpiryDateBeforeEventException (400) when expiryDate <= eventDate,
asserts DB row count unchanged after every rejection in integration tests,
and replaces all hardcoded dates in EventServiceTest with TODAY-relative
expressions derived from the fixed Clock.

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

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

@@ -2,6 +2,7 @@ package de.fete.adapter.in.web;
 
 import de.fete.application.service.EventExpiredException;
 import de.fete.application.service.EventNotFoundException;
+import de.fete.application.service.ExpiryDateBeforeEventException;
 import de.fete.application.service.ExpiryDateInPastException;
 import de.fete.application.service.InvalidTimezoneException;
 import java.net.URI;
@@ -47,6 +48,19 @@ public class GlobalExceptionHandler extends ResponseEntityExceptionHandler {
     return handleExceptionInternal(ex, problemDetail, headers, status, request);
   }
 
+  /** Handles expiry date before event date. */
+  @ExceptionHandler(ExpiryDateBeforeEventException.class)
+  public ResponseEntity<ProblemDetail> handleExpiryDateBeforeEvent(
+      ExpiryDateBeforeEventException ex) {
+    ProblemDetail problemDetail = ProblemDetail.forStatusAndDetail(
+        HttpStatus.BAD_REQUEST, ex.getMessage());
+    problemDetail.setTitle("Invalid Expiry Date");
+    problemDetail.setType(URI.create("urn:problem-type:expiry-date-before-event"));
+    return ResponseEntity.badRequest()
+        .contentType(MediaType.APPLICATION_PROBLEM_JSON)
+        .body(problemDetail);
+  }
+
   /** Handles expiry date validation failures. */
   @ExceptionHandler(ExpiryDateInPastException.class)
   public ResponseEntity<ProblemDetail> handleExpiryDateInPast(

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

@@ -32,6 +32,10 @@ public class EventService implements CreateEventUseCase, GetEventUseCase {
       throw new ExpiryDateInPastException(command.expiryDate());
     }
 
+    if (!command.expiryDate().isAfter(command.dateTime().toLocalDate())) {
+      throw new ExpiryDateBeforeEventException(command.expiryDate(), command.dateTime());
+    }
+
     var event = new Event();
     event.setEventToken(EventToken.generate());
     event.setOrganizerToken(OrganizerToken.generate());

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

@@ -0,0 +1,13 @@
+package de.fete.application.service;
+
+import java.time.LocalDate;
+import java.time.OffsetDateTime;
+
+/** Thrown when an event's expiry date is not after the event date. */
+public class ExpiryDateBeforeEventException extends RuntimeException {
+
+  /** Creates a new exception for the given dates. */
+  public ExpiryDateBeforeEventException(LocalDate expiryDate, OffsetDateTime dateTime) {
+    super("Expiry date " + expiryDate + " must be after event date " + dateTime.toLocalDate());
+  }
+}