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>

server/routes.js

@@ -1,7 +1,7 @@
 import { Router } from 'express';
 import {
   getInstances, getInstance, getDistinctStacks,
-  createInstance, updateInstance, deleteInstance, importInstances,
+  createInstance, updateInstance, deleteInstance, importInstances, getInstanceHistory,
 } from './db.js';
 
 export const router = Router();
@@ -54,6 +54,14 @@ router.get('/instances', (req, res) => {
   res.json(getInstances({ search, state, stack }));
 });
 
+// GET /api/instances/:vmid/history
+router.get('/instances/:vmid/history', (req, res) => {
+  const vmid = parseInt(req.params.vmid, 10);
+  if (!vmid) return res.status(400).json({ error: 'invalid vmid' });
+  if (!getInstance(vmid)) return res.status(404).json({ error: 'instance not found' });
+  res.json(getInstanceHistory(vmid));
+});
+
 // GET /api/instances/:vmid
 router.get('/instances/:vmid', (req, res) => {
   const vmid = parseInt(req.params.vmid, 10);