docs(adr-0026): person + user portal-side data model (proposed)

ADR-0025 left several anchors as proposed-follow-up:
- Principal.user.id / personId carry the Entra oid as a placeholder.
- StubScopeResolver returns unrestricted for everyone.
- @RequireScope's ScopableResource shape references an
  Etablissement / Delegation / Region chain that has no table.

ADR-0026 specifies the portal-side data model that closes those
stubs without dragging the upstream Pléiades / Acteurs+ syncs into
scope:

- Person golden record (stable identity; lazy-created on first
  sign-in in v1, source-stamped 'self-signin' for traceability).
- User overlay (one-to-zero-or-one with Person; portal-only state
  like lastSignInAt rides here, not on the shared Person row).
- UserScope migration confirming the ADR-0025 §"Sources of truth"
  shape.
- Region / Delegation / Etablissement hierarchy with
  externally-meaningful codes (INSEE / dept / FINESS) as primary
  keys, matching scope literals' on-the-wire shape.

Facet schemas (Salarié / Élu / Adhérent / Bénéficiaire) are
deferred to ADR-0027 alongside the Pléiades and Acteurs+ sync
that populates them - keeping the v1 migration small enough to
land + roll back cleanly.

Status: proposed. Implementation lands in two PRs once accepted:
the schema + lazy provisioner, then the PrismaScopeResolver +
admin-UI scope-seeding screen + test-tenant seed for the 19
personas.

Index updated.

docs/decisions/README.md

@@ -69,3 +69,4 @@ ADRs are listed in numerical order. To slice by topic, filter on the `Tags` colu
 | [0023](0023-charts-d3-observable-plot.md)                    | Charts + dashboards — D3 + Observable Plot wrapped in `libs/shared/charts`                                       | accepted | `frontend`, `accessibility`, `performance`                     | 2026-05-16 |
 | [0024](0024-ai-service-relay-grpc-sse-bridge.md)             | AI service relay — vendored gRPC protos, NestJS gRPC client, SSE bridge to the SPA, POC unsigned principal       | accepted | `backend`, `security`, `observability`                         | 2026-05-19 |
 | [0025](0025-authorization-model-privileges-roles-scopes.md)  | Authorization model — three orthogonal axes (privileges × functional roles × scopes), Entra-backed               | accepted | `security`, `backend`, `data`                                  | 2026-05-20 |
+| [0026](0026-person-user-portal-data-model.md)                | `Person` golden record + `User` portal-account + organisational hierarchy — portal-side data model               | proposed | `data`, `backend`, `security`                                  | 2026-05-24 |