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/db.js

@@ -43,6 +43,16 @@ function createSchema() {
     );
     CREATE INDEX IF NOT EXISTS idx_instances_state ON instances(state);
     CREATE INDEX IF NOT EXISTS idx_instances_stack ON instances(stack);
+
+    CREATE TABLE IF NOT EXISTS instance_history (
+      id         INTEGER PRIMARY KEY AUTOINCREMENT,
+      vmid       INTEGER NOT NULL,
+      field      TEXT    NOT NULL,
+      old_value  TEXT,
+      new_value  TEXT,
+      changed_at TEXT    NOT NULL DEFAULT (datetime('now'))
+    );
+    CREATE INDEX IF NOT EXISTS idx_history_vmid ON instance_history(vmid);
   `);
 }
 
@@ -99,8 +109,14 @@ export function getDistinctStacks() {
 
 // ── Mutations ─────────────────────────────────────────────────────────────────
 
+const HISTORY_FIELDS = [
+  'name', 'state', 'stack', 'vmid', 'tailscale_ip',
+  'atlas', 'argus', 'semaphore', 'patchmon', 'tailscale', 'andromeda',
+  'hardware_acceleration',
+];
+
 export function createInstance(data) {
-  return db.prepare(`
+  db.prepare(`
     INSERT INTO instances
       (name, state, stack, vmid, atlas, argus, semaphore, patchmon,
        tailscale, andromeda, tailscale_ip, hardware_acceleration)
@@ -108,10 +124,14 @@ export function createInstance(data) {
       (@name, @state, @stack, @vmid, @atlas, @argus, @semaphore, @patchmon,
        @tailscale, @andromeda, @tailscale_ip, @hardware_acceleration)
   `).run(data);
+  db.prepare(
+    `INSERT INTO instance_history (vmid, field, old_value, new_value) VALUES (?, 'created', NULL, NULL)`
+  ).run(data.vmid);
 }
 
 export function updateInstance(vmid, data) {
-  return db.prepare(`
+  const old = getInstance(vmid);
+  db.prepare(`
     UPDATE instances SET
       name=@name, state=@state, stack=@stack, vmid=@newVmid,
       atlas=@atlas, argus=@argus, semaphore=@semaphore, patchmon=@patchmon,
@@ -119,6 +139,15 @@ export function updateInstance(vmid, data) {
       hardware_acceleration=@hardware_acceleration, updated_at=datetime('now')
     WHERE vmid=@vmid
   `).run({ ...data, newVmid: data.vmid, vmid });
+  const newVmid = data.vmid;
+  const insertEvt = db.prepare(
+    `INSERT INTO instance_history (vmid, field, old_value, new_value) VALUES (?, ?, ?, ?)`
+  );
+  for (const field of HISTORY_FIELDS) {
+    const oldVal = String(old[field] ?? '');
+    const newVal = String(field === 'vmid' ? newVmid : (data[field] ?? ''));
+    if (oldVal !== newVal) insertEvt.run(newVmid, field, oldVal, newVal);
+  }
 }
 
 export function deleteInstance(vmid) {
@@ -140,6 +169,12 @@ export function importInstances(rows) {
   db.exec('COMMIT');
 }
 
+export function getInstanceHistory(vmid) {
+  return db.prepare(
+    'SELECT * FROM instance_history WHERE vmid = ? ORDER BY changed_at DESC'
+  ).all(vmid);
+}
+
 // ── Test helpers ──────────────────────────────────────────────────────────────
 
 export function _resetForTest() {