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>
