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;
 }

docker/php/php.ini

@@ -5,8 +5,8 @@ memory_limit = 256M
 max_execution_time = 60
 date.timezone = Europe/Amsterdam
 
-upload_max_filesize = 20M
-post_max_size = 21M
+upload_max_filesize = 8M
+post_max_size = 10M
 
 realpath_cache_size = 4096K
 realpath_cache_ttl = 600