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>

css/app.css

@@ -628,6 +628,30 @@ select:focus { border-color: var(--accent); }
 
 .confirm-actions { display: flex; justify-content: flex-end; gap: 10px; }
 
+/* ── HISTORY TIMELINE ── */
+.tl-item {
+  display: grid;
+  grid-template-columns: 160px 140px 1fr;
+  gap: 0 12px;
+  padding: 7px 0;
+  border-bottom: 1px solid var(--border);
+  font-size: 12px;
+  align-items: baseline;
+}
+.tl-item:last-child { border-bottom: none; }
+.tl-time   { color: var(--text3); font-size: 11px; white-space: nowrap; }
+.tl-field  { color: var(--text2); }
+.tl-change { display: flex; align-items: baseline; gap: 6px; }
+.tl-old    { color: var(--text3); text-decoration: line-through; }
+.tl-arrow  { color: var(--text3); }
+.tl-new    { color: var(--text); }
+.tl-deployed { color: var(--accent); }
+.tl-testing  { color: var(--amber); }
+.tl-degraded { color: var(--red); }
+.tl-created .tl-field  { color: var(--accent); }
+.tl-created .tl-change { color: var(--text3); }
+.tl-empty  { color: var(--text3); font-size: 12px; padding: 8px 0; }
+
 /* ── SETTINGS MODAL ── */
 #settings-modal .modal-body { padding-top: 0; }
 .settings-section { padding: 16px 0; border-bottom: 1px solid var(--border); }