Add missing @PreAuthorize to ProcessRestRepository#delete

DELETE /api/system/processes/{id} had no method-level authorization
check, unlike its siblings (findOne, findAll, findByCurrentUser). The
only check lived inside ProcessServiceImpl#delete's per-bitstream
loop, so a process with zero bitstreams (e.g. status SCHEDULED)
skipped the loop entirely and bypassed authorization, letting an
anonymous DELETE remove any process record.

Adds hasPermission(#integer, 'PROCESS', 'DELETE'), matching findOne.
Reuses the existing ProcessRestPermissionEvaluatorPlugin (owner-or-
admin check, independent of READ/WRITE/DELETE) - no new plugin needed.

Originated from security audit (2026_07_20_dq)

Author: MatusBeke

dspace-server-webapp/src/main/java/org/dspace/app/rest/repository/ProcessRestRepository.java

@@ -164,6 +164,7 @@ public BitstreamRest getProcessBitstreamByType(Integer processId, String type)
     }
 
     @Override
+    @PreAuthorize("hasPermission(#integer, 'PROCESS', 'DELETE')")
     protected void delete(Context context, Integer integer)
         throws AuthorizeException, RepositoryMethodNotImplementedException {
         try {