chore: add server provisioning migrations

- 003_add_server_tables.sql: server_orders, server_instances tables
- 003_server_sessions.sql: KV-based session tracking
- 004_add_terminated_at.sql: track instance termination time

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

migrations/004_add_terminated_at.sql

@@ -0,0 +1,5 @@
+-- Migration 004: Add terminated_at column
+-- Date: 2026-01-28
+-- Description: Add terminated_at column for server termination tracking
+
+ALTER TABLE server_orders ADD COLUMN terminated_at DATETIME;