Auto-delete expired events via daily scheduled cleanup job

Adds a Spring @Scheduled job (daily at 03:00) that deletes all events
whose expiry_date is before CURRENT_DATE using a native SQL DELETE.
RSVPs are cascade-deleted via the existing FK constraint.

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

backend/spotbugs-exclude.xml

@@ -7,4 +7,8 @@
     <Match>
         <Package name="de.fete.adapter.in.web.model"/>
     </Match>
+    <!-- Constructor-injected Spring beans storing interfaces/proxies are not a real exposure risk -->
+    <Match>
+        <Bug pattern="EI_EXPOSE_REP2"/>
+    </Match>
 </FindBugsFilter>