feat: audit log / history timeline on instance detail page

Adds an instance_history table that records every field change:
- createInstance logs a 'created' event
- updateInstance diffs old vs new and logs one row per changed field
  (name, state, stack, vmid, tailscale_ip, all service flags)
- History is stored under the new vmid when vmid changes

New endpoint: GET /api/instances/:vmid/history

The 'timestamps' section on the detail page is replaced with a
grid timeline showing timestamp | field | old → new for each event.
State changes are colour-coded (deployed=green, testing=amber,
degraded=red). Boolean service flags display as on/off.

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

tests/db.test.js

@@ -2,7 +2,7 @@ import { describe, it, expect, beforeEach } from 'vitest'
 import {
   _resetForTest,
   getInstances, getInstance, getDistinctStacks,
-  createInstance, updateInstance, deleteInstance, importInstances,
+  createInstance, updateInstance, deleteInstance, importInstances, getInstanceHistory,
 } from '../server/db.js'
 
 beforeEach(() => _resetForTest());
@@ -185,6 +185,43 @@ describe('importInstances', () => {
   });
 });
 
+// ── instance history ─────────────────────────────────────────────────────────
+
+describe('instance history', () => {
+  const base = { state: 'deployed', stack: 'production', atlas: 0, argus: 0, semaphore: 0, patchmon: 0, tailscale: 0, andromeda: 0, tailscale_ip: '', hardware_acceleration: 0 };
+
+  it('logs a created event when an instance is created', () => {
+    createInstance({ ...base, name: 'a', vmid: 1 });
+    const h = getInstanceHistory(1);
+    expect(h).toHaveLength(1);
+    expect(h[0].field).toBe('created');
+  });
+
+  it('logs changed fields when an instance is updated', () => {
+    createInstance({ ...base, name: 'a', vmid: 1 });
+    updateInstance(1, { ...base, name: 'a', vmid: 1, state: 'degraded' });
+    const h = getInstanceHistory(1);
+    const stateEvt = h.find(e => e.field === 'state');
+    expect(stateEvt).toBeDefined();
+    expect(stateEvt.old_value).toBe('deployed');
+    expect(stateEvt.new_value).toBe('degraded');
+  });
+
+  it('logs no events when nothing changes on update', () => {
+    createInstance({ ...base, name: 'a', vmid: 1 });
+    updateInstance(1, { ...base, name: 'a', vmid: 1 });
+    const h = getInstanceHistory(1).filter(e => e.field !== 'created');
+    expect(h).toHaveLength(0);
+  });
+
+  it('records history under the new vmid when vmid changes', () => {
+    createInstance({ ...base, name: 'a', vmid: 1 });
+    updateInstance(1, { ...base, name: 'a', vmid: 2 });
+    expect(getInstanceHistory(2).some(e => e.field === 'vmid')).toBe(true);
+    expect(getInstanceHistory(1).filter(e => e.field !== 'created')).toHaveLength(0);
+  });
+});
+
 // ── Test environment boot isolation ───────────────────────────────────────────
 
 describe('test environment boot isolation', () => {