Raise upload limit to 8 MB

- docker/php/php.ini: upload_max_filesize 8M / post_max_size 10M
- docker/nginx/default.conf: client_max_body_size 10M
- ArtworkType: Image maxSize 8M with a clear message + help text

(local C:\php\php.ini was bumped from 2M the same way — not in the repo)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docker/nginx/default.conf

@@ -51,5 +51,5 @@ server {
     error_log /var/log/nginx/dmtools_error.log;
     access_log /var/log/nginx/dmtools_access.log;
 
-    client_max_body_size 20M;
+    client_max_body_size 10M;
 }