Add model card resolver to VFolderGQL

[fregataa]

Background

ModelCardGQL.vfolder exposes the linked VFolder from a model card (api/gql/model_card/types.py:221-235), but the reverse direction is not exposed. There is no way to obtain the matching model card from a vfolder via either REST v2 or GraphQL.

Findings:

• VFolderGQL (api/gql/vfolder_v2/types/node.py) has no model_card field.

• VFolderNode REST v2 DTO (common/dto/manager/v2/vfolder/response.py:57) has no model_card field.

• ModelCardFilter (common/dto/manager/v2/model_card/request.py) has no vfolder_id filter, so even an indirect lookup via model card search by vfolder ID is impossible.

• Clients today must list all model cards in a project and match by vfolder_id field on the client side.

Goal

Add a model_card field to VFolderGQL that returns the linked ModelCardGQL | None, mirroring the existing ModelCardGQL.vfolder resolver.

Scope

• Add a ModelCardLoader (or extend an existing data loader) keyed by vfolder_id to avoid N+1.

• Add model_card field on VFolderGQL using strawberry.lazy() for the cross-entity reference.

• Update VFolderNode REST v2 DTO if parity is desired (optional - GQL-first is acceptable).

• Add tests covering: vfolder with no card, vfolder with card, RBAC visibility (caller can see vfolder but not the card).

Out of scope

• Adding vfolder_id filter to ModelCardFilter (separate concern).

• REST v2 inline expansion (can be added later if demand arises).

References

• api/gql/CLAUDE.md — N+1 prevention rule, lazy import pattern.

• api/gql/model_card/types.py:221-235 — existing forward-direction resolver as reference.

JIRA Issue: BA-5947