Make Solr heap tunable via SOLR_HEAP env var

Default stays 4g, so no behavior change unless SOLR_HEAP is set.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: vidiecan

docker/docker-compose-rest.yml

@@ -174,7 +174,7 @@ services:
       cp -r /opt/solr/server/solr/configsets/search/* search
       precreate-core statistics /opt/solr/server/solr/configsets/statistics
       cp -r /opt/solr/server/solr/configsets/statistics/* statistics
-      exec solr -p 8983 -f -m 4g
+      exec solr -p 8983 -f -m ${SOLR_HEAP:-4g}
 volumes:
   # Commented out because there are a lot of files in the assetstore
   assetstore: