fix: clear instance history on delete and import

deleteInstance now removes history rows for that vmid before removing
the instance. importInstances clears all history before replacing
instances. Prevents stale history appearing when a vmid is reused.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/db.test.js

@@ -164,6 +164,19 @@ describe('deleteInstance', () => {
     expect(getInstance(1)).toBeNull();
     expect(getInstance(2)).not.toBeNull();
   });
+
+  it('clears history for the deleted instance', () => {
+    createInstance({ ...base, name: 'a', vmid: 1 });
+    deleteInstance(1);
+    expect(getInstanceHistory(1)).toHaveLength(0);
+  });
+
+  it('does not clear history for other instances', () => {
+    createInstance({ ...base, name: 'a', vmid: 1 });
+    createInstance({ ...base, name: 'b', vmid: 2 });
+    deleteInstance(1);
+    expect(getInstanceHistory(2).length).toBeGreaterThan(0);
+  });
 });
 
 // ── importInstances ───────────────────────────────────────────────────────────
@@ -183,6 +196,12 @@ describe('importInstances', () => {
     importInstances([]);
     expect(getInstances()).toEqual([]);
   });
+
+  it('clears history for all replaced instances', () => {
+    createInstance({ ...base, name: 'old', vmid: 1 });
+    importInstances([{ ...base, name: 'new', vmid: 2 }]);
+    expect(getInstanceHistory(1)).toHaveLength(0);
+  });
 });
 
 // ── instance history ─────────────────────────────────────────────────────────