feat(auth): authorization catalogues + Principal builder skeleton (ADR-0025)

Per ADR-0025's implementation phasing, lands the closed-set
catalogues + the OIDC-callback hook that composes the
session-resident Principal. No new guards yet.

- libs/shared/auth: framework-agnostic lib hosting
  authorization.types.ts (4 privileges + 24 functional roles + 6
  scope kinds + Principal type) and entra-group-to-role.ts
  (validated GUID -> slug resolver). 17 unit tests against the
  catalogues + resolver contracts.
- BFF: PrincipalBuilder composes the three axes once at sign-in;
  ScopeResolver seam (StubScopeResolver returns unrestricted in
  v1, replaced by a Prisma-backed implementation when ADR-0026's
  user_scopes table lands). 19 persona-driven tests covering the
  test tenant's full provisioning + edge cases (unknown
  privilege drift, unknown group GUID, empty permissions).
- AuthService extracts the Entra "groups" claim; AuthenticatedUser
  grows the groups: readonly string[] field.
- SessionEstablisher builds + persists the principal alongside
  the legacy user shape (additive — guards consuming user.oid
  unchanged).
- ENTRA_GROUP_MAP_PATH env var + load-entra-group-map.ts read
  the tenant-private map from infra/<env>-tenant.entra.json
  (gitignored; .example.json committed with the schema).
- ADR-0025 path references updated from libs/feature/auth
  (Angular-scoped, scope:portal-shell) to libs/shared/auth
  (consumable by both BFF and SPA).
- notes/entra-groups-claim-activation.md: operator runbook for
  the "Token configuration -> Add groups claim" step in the
  Entra admin centre.

Test plan:
- pnpm nx affected -t lint test build : 13 projects green
  (497 BFF tests + 17 shared-auth tests + 1 shared-util test +
  full lint + full build).

libs/shared/auth/README.md

@@ -0,0 +1,24 @@
+# shared-auth
+
+Framework-agnostic authorization primitives shared across `portal-bff` and
+`portal-shell` per [ADR-0025](../../../docs/decisions/0025-authorization-model-privileges-roles-scopes.md):
+
+- **Catalogues** (`authorization.types.ts`) — closed-set lists of the four
+  privileges (`Portal.*` Entra app roles), the 24 functional roles
+  (`apf-role-*` Entra security groups), and the six scope kinds. Adding an
+  entry is an ADR amendment, not a code change in isolation.
+- **`Principal` type** — the session-resident shape consumed by route
+  guards (`@RequirePrivilege`, `@RequireRole`, `@RequireScope`) and projected
+  to the AI service's flat `roles[]` contract.
+- **`EntraGroupToRoleResolver`** — turns the tenant-specific Entra group
+  GUIDs carried in the OIDC `groups` claim into the catalogue's role slugs.
+  Configured at boot from `infra/<env>-tenant.entra.json` (gitignored —
+  GUIDs are tenant-private).
+
+## Building
+
+Run `pnpm nx build shared-auth` to build the library.
+
+## Running unit tests
+
+Run `pnpm nx test shared-auth` to execute the unit tests via [Vitest](https://vitest.dev/).