feat(users): add Person + User + UserScope + lazy provisioner (ADR-0026 PR 1)

Schema: Person golden record + User portal-account overlay (1-to-0-or-1)
+ UserScope (the table behind the ADR-0025 scope axis). All in the
public schema. UserScope.value is opaque (no FK to ADR-0027 tables -
historical scope rows survive structure decommissioning).

Migration: hand-written, follows the existing convention. FKs with
correct ON DELETE actions (RESTRICT on User.personId for the required
relation, CASCADE on UserScope.userId via explicit onDelete: Cascade
in the schema).

Catalogue: PERSON_SOURCES = [self-signin, admin-ui, seed] as const
under apps/portal-bff/src/users/person-source.ts. Defense in depth =
TS type union + drift gate scanner extension (property literal
"source: 'X'" in files importing person-source.ts).

PersonAndUserProvisioner: blocking lazy-create at first OIDC callback.
Fast path = findUnique by entraOid + update lastSignInAt. Cold path =
nested create of Person + User in one transaction. Race-condition
handling: catches P2002 on User.entraOid unique constraint and re-runs
ensureUser. Defense-in-depth isPersonSource check on the constant.

PrincipalBuilder signature: build(user, identity: { userId, personId }).
Principal.user.{id, personId} populated from real UUIDs. ScopeResolver
seam moved from { entraOid } to { userId } so PR 2's PrismaScopeResolver
can key queries on User.id.

SessionEstablisher: new constructor arg personUserProvisioner.
establish() calls ensureUser BEFORE principalBuilder.build so the
identity is available. Entra preferred_username (= AuthenticatedUser.
username) maps to Person.email per ADR-0026 lifecycle. Provisioner
failure short-circuits the whole flow before save / cookie / audit /
directory.

UserDirectoryService stays as the ADR-0020 admin-list cache - folding
it into Person+User is a follow-up after ADR-0026 PR 2 stabilises.

Local verification: drift gate clean (4 / 24 / 7 / 3), 29 drift-gate
spec tests passing, portal-bff lint 0 errors, 766 portal-bff specs
passing.

apps/portal-bff/prisma/schema.prisma

@@ -106,6 +106,118 @@ model UserDirectoryEntry {
   @@index([username])
 }
 
+// ============================================================
+// Identity model (per ADR-0026)
+// ============================================================
+//
+// `Person` golden record + `User` portal-account overlay (one-to-zero-
+// or-one). Distinct from `UserDirectoryEntry` above — that one is the
+// ADR-0020 sign-in cache keyed on Entra `oid`; these are the portal's
+// stable identity model keyed on UUIDs and form the basis for the
+// `@RequireScope` Prisma resolver landing in ADR-0026 PR 2.
+//
+// `Person` exists whether or not the human ever signs in (workforce
+// pre-provisioning, dossier bénéficiaires, alumni). `User` is the
+// portal-access overlay, lazy-created at first OIDC callback by
+// `PersonAndUserProvisioner.ensureUser`. `UserScope` backs the ADR-0025
+// scope axis with opaque `value` strings referencing ADR-0027's
+// `Structure.code` / `Delegation.code` / `Region.code` — no FK at the
+// DB level so historical scope rows survive structure decommissioning.
+
+model Person {
+  id         String   @id @default(uuid()) @db.Uuid
+  // PII — firstName + lastName are subject to ADR-0013 redaction rules
+  // when emitted to logs.
+  firstName  String   @map("first_name")
+  lastName   String   @map("last_name")
+  // Primary contact email. Indexed for operator lookup but NOT unique
+  // — two distinct humans genuinely can share emails (shared family
+  // alias, generic info@ at a small partner organisation, error in an
+  // upstream feed). ADR-0029's reconciliation flow surfaces candidate
+  // duplicates for operator confirmation rather than auto-merging.
+  email      String?
+  // Closed-set catalogue, drift-gated. See
+  // `apps/portal-bff/src/users/person-source.ts` for the legal values.
+  // v1: 'self-signin' / 'admin-ui' / 'seed'. ADR-0029 adds 'pleiades'
+  // and 'acteurs-plus'.
+  source     String
+  // Upstream-system identifier (Pléiades matricule, Acteurs+ id).
+  // Nullable in v1 — populated by ADR-0029's syncs.
+  externalId String?  @map("external_id")
+  createdAt  DateTime @default(now()) @map("created_at") @db.Timestamptz(6)
+  updatedAt  DateTime @updatedAt @map("updated_at") @db.Timestamptz(6)
+
+  // Back-ref. NULL when the Person has no portal account (workforce
+  // pre-provisioning, dossier bénéficiaire, alumni).
+  user User?
+
+  @@map("persons")
+  @@schema("public")
+  @@index([source])
+  @@index([externalId])
+  @@index([email])
+}
+
+model User {
+  id String @id @default(uuid()) @db.Uuid
+  // 1-to-1 with Person. The unique constraint on personId enforces
+  // "at most one User per Person"; back-ref `Person.user` is the
+  // other half.
+  personId String @unique @map("person_id") @db.Uuid
+  person   Person @relation(fields: [personId], references: [id])
+  // Entra object id — stable per-user identifier inside the tenant.
+  // Unique because two separate Person+User pairs cannot share an
+  // Entra identity.
+  entraOid String @unique @map("entra_oid")
+  // Entra tenant id. Stored alongside the oid so a future dual-
+  // audience activation (ADR-0008) can disambiguate.
+  tenantId String @map("tenant_id")
+  // Refreshed by `PersonAndUserProvisioner.ensureUser` on every
+  // sign-in. Surfaced in the future /admin/users/:id screen.
+  lastSignInAt DateTime? @map("last_sign_in_at") @db.Timestamptz(6)
+  createdAt    DateTime  @default(now()) @map("created_at") @db.Timestamptz(6)
+  updatedAt    DateTime  @updatedAt @map("updated_at") @db.Timestamptz(6)
+
+  scopes UserScope[]
+
+  @@map("users")
+  @@schema("public")
+}
+
+model UserScope {
+  id     String @id @default(uuid()) @db.Uuid
+  userId String @map("user_id") @db.Uuid
+  // CASCADE so revoking a User wipes their scope rows in one delete.
+  user User @relation(fields: [userId], references: [id], onDelete: Cascade)
+  // One of the six ADR-0025 scope kinds: 'self' / 'etablissement' /
+  // 'delegation' / 'region' / 'siege' / 'unrestricted'. Not pulled
+  // from a Prisma enum because the value's semantics belong to the
+  // `shared-auth` catalogue, not the database; the drift gate
+  // already enforces the closed set at the call sites.
+  kind String
+  // Per-kind payload — semantics owned by ADR-0027. For 'etablissement'
+  // the value is a Structure.code; for 'delegation' it is a
+  // Delegation.code; for 'region' it is a Region.code. Empty string
+  // for 'self' / 'siege' / 'unrestricted'. NO FK to ADR-0027 tables —
+  // historical scope rows survive structure decommissioning; the
+  // admin-UI write path (ADR-0026 PR 2) validates at insert time.
+  value String @default("")
+  // Provenance, same catalogue posture as Person.source. v1:
+  // 'admin-ui' / 'seed'. ADR-0029 adds upstream-sync values and a
+  // reconciliation policy.
+  source    String
+  createdAt DateTime @default(now()) @map("created_at") @db.Timestamptz(6)
+  // When non-null and past, the row is ignored at sign-in. Supports
+  // interim-director-for-two-months patterns and admin UI scope
+  // revocations.
+  expiresAt DateTime? @map("expires_at") @db.Timestamptz(6)
+
+  @@map("user_scopes")
+  @@schema("public")
+  @@unique([userId, kind, value])
+  @@index([userId])
+}
+
 // ============================================================
 // Organisational hierarchy (per ADR-0027)
 // ============================================================