feat(ST-702): vendor Scrum4Me schema via submodule + sync script

- Add vendor/scrum4me as a git submodule (read-only source of truth)
- scripts/sync-schema.sh copies schema.prisma into prisma/, stripping
  the upstream prisma-erd-generator block we don't ship
- Track the synced schema in git so a fresh clone works after
  'git submodule update --init && npm install'
- postinstall runs 'prisma generate' so @prisma/client is ready

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -16,6 +16,7 @@
     "build": "tsc",
     "start": "node dist/index.js",
     "prisma:generate": "prisma generate",
+    "postinstall": "prisma generate || true",
     "typecheck": "tsc --noEmit",
     "sync-schema": "bash scripts/sync-schema.sh"
   },