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>

ui/src/api/queries/admin/environments.ts

@@ -8,6 +8,7 @@ export interface Environment {
   production: boolean;
   enabled: boolean;
   defaultContainerConfig: Record<string, unknown>;
+  jarRetentionCount: number | null;
   createdAt: string;
 }
 
@@ -66,6 +67,18 @@ export function useUpdateDefaultContainerConfig() {
   });
 }
 
+export function useUpdateJarRetention() {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: ({ id, jarRetentionCount }: { id: string; jarRetentionCount: number | null }) =>
+      adminFetch<Environment>(`/environments/${id}/jar-retention`, {
+        method: 'PUT',
+        body: JSON.stringify({ jarRetentionCount }),
+      }),
+    onSuccess: () => qc.invalidateQueries({ queryKey: ['admin', 'environments'] }),
+  });
+}
+
 export function useDeleteEnvironment() {
   const qc = useQueryClient();
   return useMutation({