fix(portal-bff): drop strict amr check — flow blocked when Entra omits the claim (#108)

## Bug

After a real sign-in against the Entra tenant, the callback rejected the flow with:

```
{"context":"AuthCallback","event":"auth.flow_error","failure":{"kind":"amr-missing"}}
```

The user landed on the SPA with `?auth_error=amr-missing` instead of authenticated. Every dev sign-in is blocked.

## Root cause

PR #107's `amr-missing` guard misread ADR-0011's intent. `amr` is an **optional** claim in Entra ID tokens: it's populated for fresh interactive sign-ins where Conditional Access asked for an MFA method, and frequently absent for SSO / refresh flows or in tenants where no CA policy is configured on the app registration. Rejecting tokens on empty `amr` blocks every legitimate sign-in against such a tenant.

ADR-0011 actually specifies:
- **Conditional Access** (org-side) is the enforcement layer for "MFA happened".
- The **`@RequireMfa({ freshness: 600 })`** decorator (designed-in, no v1 consumer) is what guards sensitive routes.
- The BFF surfaces `amr` through the audit log and the future guard, not as a callback precondition.

## Fix

- **[`auth.errors.ts`](apps/portal-bff/src/auth/auth.errors.ts)**: drop the `amr-missing` variant from the `AuthCodeFlowError` discriminator. Three failure modes left: `state-mismatch`, `flow-expired`, `token-exchange-failed`. MSAL's ID-token validation (signature, issuer, audience, exp, nbf) is the real gate at this stage.
- **[`auth.service.ts`](apps/portal-bff/src/auth/auth.service.ts)**: `toAuthenticatedUser` keeps extracting `amr` and passing it through (as a possibly-empty string array) so the structured log line and the future `@RequireMfa` guard still see it. The strict `if (amr.length === 0) throw` is replaced by a comment explaining the new shape.
- **[`auth.service.spec.ts`](apps/portal-bff/src/auth/auth.service.spec.ts)**: the `'throws amr-missing'` test becomes `'returns the user even when the ID token has no amr claim'` — asserts the array passes through empty rather than blocking the flow.

## Verification

- `nx run-many -t lint test build --projects=portal-bff` — green. **52/52 specs**.
- Manual smoke: end-to-end sign-in against the live tenant now lands cleanly on the SPA; Pino's `auth.signed_in` log shows the resolved identity with `amr` (often `[]` until CA is configured on the org side).

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #108

apps/portal-bff/src/auth/auth.service.spec.ts

@@ -129,12 +129,19 @@ describe('AuthService.completeAuthCodeFlow', () => {
     ).rejects.toMatchObject({ failure: { kind: 'flow-expired' } });
   });
 
-  it('throws amr-missing when the ID token has no amr claim', async () => {
+  it('returns the user even when the ID token has no `amr` claim (Entra optional)', async () => {
     const acquireTokenByCode = jest.fn().mockResolvedValue(makeAuthResult({ amr: [] }));
     const { service } = makeService({ acquireTokenByCode });
-    await expect(
-      service.completeAuthCodeFlow('code', PRE_AUTH_OK.state, PRE_AUTH_OK, PRE_AUTH_OK.createdAt),
-    ).rejects.toMatchObject({ failure: { kind: 'amr-missing' } });
+    const user = await service.completeAuthCodeFlow(
+      'code',
+      PRE_AUTH_OK.state,
+      PRE_AUTH_OK,
+      PRE_AUTH_OK.createdAt,
+    );
+    // `amr` flows through as an empty array; MFA enforcement is
+    // Conditional Access's job per ADR-0011, not this code path.
+    expect(user.amr).toEqual([]);
+    expect(user.oid).toBe('user-oid');
   });
 
   it('throws token-exchange-failed when MSAL throws', async () => {