docs(backend): track sprint-0 follow-ups + flag integration migration gap

- `tasks/todo.md`: B0.5 description updated (re2 hard dep, no fallback);
  add a "Backend follow-ups (sprint 1+)" section with M1-M7 + N1-N6 from
  the code-review verdict.
- `CHANGELOG.md`: backend skeleton bullets refreshed (no re fallback,
  streaming blob store, audit + scope on CRUD, 56 unit tests); new
  "Code-review remediation" subsection lists B1 / MA1-MA6 / N4 / N6 / M8
  with one-line rationale each.
- `tests/integration/conftest.py`: leave `db.create_all()` in place but
  add an inline TODO (N6) pointing at the Alembic switchover that will
  exercise the F11 seed + audit-log role grants in CI.

backend/tests/integration/conftest.py

@@ -37,6 +37,9 @@ def app(postgres_dsn: str, monkeypatch: pytest.MonkeyPatch):
 
     application = create_app()
     with application.app_context():
+        # TODO (N6 follow-up, sprint 1): run Alembic migrations instead of
+        # db.create_all() so the integration tests exercise the real schema
+        # including the audit_log role grants and the F11 seed.
         db.create_all()
         yield application
         db.session.remove()