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/pages/Admin/EnvironmentsPage.tsx

@@ -21,6 +21,7 @@ import {
   useUpdateEnvironment,
   useDeleteEnvironment,
   useUpdateDefaultContainerConfig,
+  useUpdateJarRetention,
 } from '../../api/queries/admin/environments';
 import type { Environment } from '../../api/queries/admin/environments';
 import styles from './UserManagement.module.css';
@@ -44,6 +45,7 @@ export default function EnvironmentsPage() {
   const updateEnv = useUpdateEnvironment();
   const deleteEnv = useDeleteEnvironment();
   const updateDefaults = useUpdateDefaultContainerConfig();
+  const updateRetention = useUpdateJarRetention();
 
   const selected = useMemo(
     () => environments.find((e) => e.id === selectedId) ?? null,
@@ -291,6 +293,15 @@ export default function EnvironmentsPage() {
                   toast({ title: 'Failed to update defaults', variant: 'error', duration: 86_400_000 });
                 }
               }} saving={updateDefaults.isPending} />
+
+              <JarRetentionSection environment={selected} onSave={async (count) => {
+                try {
+                  await updateRetention.mutateAsync({ id: selected.id, jarRetentionCount: count });
+                  toast({ title: 'Retention policy updated', variant: 'success' });
+                } catch {
+                  toast({ title: 'Failed to update retention', variant: 'error', duration: 86_400_000 });
+                }
+              }} saving={updateRetention.isPending} />
             </>
           ) : null
         }
@@ -389,3 +400,71 @@ function DefaultResourcesSection({ environment, onSave, saving }: {
     </>
   );
 }
+
+// ── JAR Retention Policy ────────────────────────────────────────────
+
+function JarRetentionSection({ environment, onSave, saving }: {
+  environment: Environment;
+  onSave: (count: number | null) => Promise<void>;
+  saving: boolean;
+}) {
+  const current = environment.jarRetentionCount;
+  const [editing, setEditing] = useState(false);
+  const [unlimited, setUnlimited] = useState(current === null);
+  const [count, setCount] = useState(String(current ?? 5));
+
+  useEffect(() => {
+    setUnlimited(environment.jarRetentionCount === null);
+    setCount(String(environment.jarRetentionCount ?? 5));
+    setEditing(false);
+  }, [environment.id]);
+
+  function handleCancel() {
+    setUnlimited(current === null);
+    setCount(String(current ?? 5));
+    setEditing(false);
+  }
+
+  async function handleSave() {
+    await onSave(unlimited ? null : Math.max(1, parseInt(count) || 5));
+    setEditing(false);
+  }
+
+  return (
+    <>
+      <SectionHeader>JAR Retention</SectionHeader>
+      <p className={styles.inheritedNote}>
+        Old JAR versions are cleaned up nightly. Currently deployed versions are never deleted.
+      </p>
+      <div className={styles.metaGrid}>
+        <span className={styles.metaLabel}>Policy</span>
+        {editing ? (
+          <div style={{ display: 'flex', alignItems: 'center', gap: 8 }}>
+            <Toggle checked={!unlimited} onChange={() => setUnlimited(!unlimited)} />
+            {unlimited
+              ? <span>Keep all versions (unlimited)</span>
+              : <>
+                  <span>Keep last</span>
+                  <Input value={count} onChange={(e) => setCount(e.target.value)} style={{ width: 60 }} />
+                  <span>versions</span>
+                </>}
+          </div>
+        ) : (
+          <span className={styles.metaValue}>
+            {current === null ? 'Unlimited (no cleanup)' : `Keep last ${current} versions`}
+          </span>
+        )}
+      </div>
+      <div style={{ marginTop: 8, display: 'flex', gap: 8, justifyContent: 'flex-end' }}>
+        {editing ? (
+          <>
+            <Button size="sm" variant="ghost" onClick={handleCancel}>Cancel</Button>
+            <Button size="sm" variant="primary" onClick={handleSave} loading={saving}>Save</Button>
+          </>
+        ) : (
+          <Button size="sm" variant="secondary" onClick={() => setEditing(true)}>Edit Policy</Button>
+        )}
+      </div>
+    </>
+  );
+}