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>

js/ui.js

@@ -99,8 +99,21 @@ async function filterInstances() {
 
 // ── Detail Page ───────────────────────────────────────────────────────────────
 
+const BOOL_FIELDS = ['atlas','argus','semaphore','patchmon','tailscale','andromeda','hardware_acceleration'];
+
+function stateClass(field, val) {
+  if (field !== 'state') return '';
+  return { deployed: 'tl-deployed', testing: 'tl-testing', degraded: 'tl-degraded' }[val] ?? '';
+}
+
+function fmtHistVal(field, val) {
+  if (val == null || val === '') return '—';
+  if (BOOL_FIELDS.includes(field)) return val === '1' ? 'on' : 'off';
+  return esc(val);
+}
+
 async function renderDetailPage(vmid) {
-  const inst = await getInstance(vmid);
+  const [inst, history] = await Promise.all([getInstance(vmid), getInstanceHistory(vmid)]);
   if (!inst) { navigate('dashboard'); return; }
   currentVmid = vmid;
 
@@ -133,10 +146,26 @@ async function renderDetailPage(vmid) {
     </div>
   `).join('');
 
-  document.getElementById('detail-timestamps').innerHTML = `
-    <div class="kv-row"><span class="kv-key">created</span><span class="kv-val">${fmtDateFull(inst.created_at)}</span></div>
-    <div class="kv-row"><span class="kv-key">updated</span><span class="kv-val">${fmtDateFull(inst.updated_at)}</span></div>
-  `;
+  document.getElementById('detail-timestamps').innerHTML = history.length
+    ? history.map(e => {
+        if (e.field === 'created') return `
+          <div class="tl-item tl-created">
+            <span class="tl-time">${fmtDateFull(e.changed_at)}</span>
+            <span class="tl-field">created</span>
+            <span class="tl-change">—</span>
+          </div>`;
+        return `
+          <div class="tl-item">
+            <span class="tl-time">${fmtDateFull(e.changed_at)}</span>
+            <span class="tl-field">${esc(e.field)}</span>
+            <span class="tl-change">
+              <span class="tl-old">${fmtHistVal(e.field, e.old_value)}</span>
+              <span class="tl-arrow">→</span>
+              <span class="tl-new ${stateClass(e.field, e.new_value)}">${fmtHistVal(e.field, e.new_value)}</span>
+            </span>
+          </div>`;
+      }).join('')
+    : '<div class="tl-empty">no history yet</div>';
 
   document.getElementById('detail-edit-btn').onclick   = () => openEditModal(inst.vmid);
   document.getElementById('detail-delete-btn').onclick = () => confirmDeleteDialog(inst);