fix: add proper 409 errors to demo

Author: stark-cybertech

server/matching/src/main/java/meet_at_mensa/matching/controller/MatchingController.java

@@ -223,6 +223,12 @@ public ResponseEntity<Group> postApiV2MatchingDemo(MatchRequestNew matchRequestN
             // return 200 with MatchRequestCollection if found
             return ResponseEntity.ok(demoGroup);
 
+        // 409
+        } catch (RequestOverlapException e) {
+            
+            // return 409 if user already has a request on that day
+            return ResponseEntity.status(HttpStatus.CONFLICT).build();
+
         // 500
         } catch (Exception e) {