db: auto-add missing model columns on startup (fix backupschedule.agent_id) (0.38.3)

create_all never ALTERs an existing table, so installs predating the
backupschedule.agent_id column kept the old schema and any ORM query
naming it failed with "no such column" — which the new fleet dashboard
(and the schedules list / scheduler loop) hit. init_db now diffs each
mapped table against the live schema and ADD COLUMNs the missing
nullable/defaulted ones. Idempotent and self-healing for similar drift.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.38.2",
+  "version": "0.38.3",
   "type": "module",
   "scripts": {
     "dev": "vite",