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>

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "vendor/scrum4me"]
+	path = vendor/scrum4me
+	url = https://github.com/madhura68/Scrum4Me.git