Module-level session helpers move into sqlmodel_repo/_helpers.py. __init__.py re-exports them so external import paths (decnet.web.db.sqlmodel_repo._safe_session) keep resolving.
Pure rename — the old monolithic 3505-line file becomes decnet/web/db/sqlmodel_repo/__init__.py. No code changes. Subsequent commits will extract per-domain mixins out of __init__.py to mirror the topical layout used by decnet/web/db/models/.
