fix(config-backup): a self-refreshed discovery timestamp is not a change

custom-model-hosts.json carries a lastDiscoveredAt per host that the 5-minute model
discovery poll rewrites, so the content hash never matched the latest snapshot and a
snapshot was written every tick. At the default keep count of 20 that left about 100
minutes of history and evicted every useful older copy.

The change check now looks through volatile keys per tracked file. The snapshot still
stores the file as written and restore still verifies against its own digest.

test/config-backup.test.ts

@@ -124,6 +124,35 @@ describe('createConfigBackup', () => {
     expect(listConfigBackups(backups)).toHaveLength(2);
   });
 
+  it('does not count a self-refreshed discovery timestamp as a change', () => {
+    const hosts = (stamp: string, model = 'm1') =>
+      JSON.stringify({ hosts: [{ id: 'h', models: [{ id: model }], lastDiscoveredAt: stamp }] });
+    put('custom-model-hosts.json', hosts('2026-10-12T08:44:22.026Z'));
+    const first = createConfigBackup({ dataDir: data, backupDir: backups, reason: 'auto', now: T0 });
+
+    // The 5-minute discovery poll rewrites only the timestamp: still the same state.
+    put('custom-model-hosts.json', hosts('2026-10-12T08:49:22.025Z'));
+    const again = createConfigBackup({ dataDir: data, backupDir: backups, reason: 'auto', now: T0 + 1000 });
+    expect(again).toEqual({ status: 'unchanged', id: (first as { id: string }).id });
+    expect(listConfigBackups(backups)).toHaveLength(1);
+
+    // A real edit next to a refreshed timestamp is still a change, stored exactly as written.
+    const edited = hosts('2026-10-12T08:54:22.025Z', 'm2');
+    put('custom-model-hosts.json', edited);
+    const third = createConfigBackup({ dataDir: data, backupDir: backups, reason: 'auto', now: T0 + 2000 });
+    expect(third.status).toBe('created');
+    expect(readFileSync(join(backups, (third as { id: string }).id, 'custom-model-hosts.json'), 'utf8')).toBe(edited);
+  });
+
+  it('compares a tracked file that is not valid JSON by its bytes', () => {
+    put('custom-model-hosts.json', 'not json lastDiscoveredAt 1');
+    createConfigBackup({ dataDir: data, backupDir: backups, reason: 'auto', now: T0 });
+    put('custom-model-hosts.json', 'not json lastDiscoveredAt 2');
+    expect(createConfigBackup({ dataDir: data, backupDir: backups, reason: 'auto', now: T0 + 1000 }).status).toBe(
+      'created'
+    );
+  });
+
   it('force makes a snapshot even when unchanged, and two in one second get distinct ids', () => {
     put('settings.json', '{}');
     createConfigBackup({ dataDir: data, backupDir: backups, reason: 'a', now: T0 });