fix(api): GET /documents crashes with 500 when created_at column is NULL in legacy DB rows #294

Summary

_to_document_schema in backend/app/api/documents.py at line ~136 passes record.created_at (which is str | None in DocumentRecord) to Document(created_at=...) where created_at: str is non-optional in the Pydantic schema. When the DB row has created_at=NULL (e.g. legacy rows from migration 001 before onupdate was added), Pydantic raises ValidationError and the GET /documents endpoint returns unhandled HTTP 500.

Fix

backend/app/api/documents.py: either make Document.created_at optional (str | None = None) or coerce None to an empty string / ISO timestamp default in _to_document_schema.

Impact

Any tenant that has documents created before the created_at column was added (migration 001 rows without onupdate DEFAULT) gets HTTP 500 on GET /documents.