feat(backend): limit multipart upload to 5 MB file / 6 MB request

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

backend/src/main/resources/application.yml

@@ -19,5 +19,10 @@ spring:
     enabled: true
     locations: classpath:db/migration
 
+  servlet:
+    multipart:
+      max-file-size: 5MB
+      max-request-size: 6MB
+
 server:
   port: 8080