feat: JAR retention policy with nightly cleanup job

Per-environment "keep last N versions" setting (default 5, null for
unlimited). Nightly scheduled job at 03:00 deletes old versions from
both database and disk, skipping any version that is currently deployed.

Full stack:
- V6 migration: adds jar_retention_count column to environments
- Environment record, repository, service, admin controller endpoint
- JarRetentionJob: @Scheduled nightly, iterates environments and apps
- UI: retention policy editor on admin Environments page with
  toggle between limited/unlimited and version count input
- AppVersionRepository.delete() for version cleanup

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-app/src/main/resources/db/migration/V6__jar_retention_policy.sql

@@ -0,0 +1 @@
+ALTER TABLE environments ADD COLUMN jar_retention_count INTEGER DEFAULT 5;