ci: add migration integrity gate for migration-relevant changes

docs/03-development.md

@@ -1,3 +1,23 @@
 # Development workflow
 
-Placeholder: see root `README.md` for current setup steps.
+## Migration integrity gate (CI)
+
+CI enforces a migration integrity gate to prevent merge-time schema breakages.
+
+### What it validates
+
+- Alembic migrations can apply from a clean Postgres database (`upgrade head`)
+- Alembic revision graph resolves to a head revision after migration apply
+- On migration-relevant PRs, CI also checks that model changes are accompanied by migration updates
+
+If any of these checks fails, CI fails and the PR is blocked.
+
+### Local reproduction
+
+From repo root:
+
+```bash
+make backend-migration-check
+```
+
+This command starts a temporary Postgres container, runs migration checks, and cleans up the container.