Address second round of Copilot review comments

- CHANGELOG.md: rename the premature "10.2.5" heading to "Unreleased",
  matching the repo convention where the release commit assigns the
  version number (see 855d267 for 10.2.4).
- docker-compose.yml: make the dev-stack Elasticsearch heap overridable
  via ES_JAVA_OPTS in .env (default unchanged at 2g), using the compose
  file's existing ${VAR:-default} idiom, for smaller machines.

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

CHANGELOG.md

@@ -1,6 +1,6 @@
 # Changelog
 
-## 10.2.5
+## Unreleased
 
 ### New features
 

docker-compose.yml

@@ -13,7 +13,8 @@ services:
       - xpack.license.self_generated.type=basic
       # Without an explicit heap, ES sizes it to 50% of host RAM and mlocks it
       # (bootstrap.memory_lock), which OOM-kills the container on large hosts.
-      - ES_JAVA_OPTS=-Xms2g -Xmx2g
+      # Override via ES_JAVA_OPTS in .env for smaller machines.
+      - ES_JAVA_OPTS=${ES_JAVA_OPTS:--Xms2g -Xmx2g}
     ports:
       - "127.0.0.1:9200:9200"
     ulimits: