feat(portal-bff): /auth/callback route — token exchange + amr check (#107)

## Summary

Fourth step of ADR-0009 wiring. Closes the OIDC round-trip on the BFF side (modulo session persistence — that's the next PR per ADR-0010). Entra now redirects the user back to `GET /api/auth/callback`; the BFF verifies the state, exchanges the code for tokens via MSAL's `acquireTokenByCode`, runs the ADR-0011 `amr` sanity-check, logs the resolved identity to Pino, clears the single-use pre-auth cookie, and 302s the user back to the SPA.

## What lands

- **[`auth.errors.ts`](apps/portal-bff/src/auth/auth.errors.ts)** — discriminated-union `AuthCodeFlowError` (`state-mismatch` / `flow-expired` / `amr-missing` / `token-exchange-failed`) + `AuthCodeFlowException` wrapper. The `kind` field doubles as the `?auth_error=<code>` query param on the SPA-bound redirect so the front-end can render an exact message without duplicating the string set.
- **[`AuthService.completeAuthCodeFlow(code, state, preAuth, now?)`](apps/portal-bff/src/auth/auth.service.ts)** — verifies state binding, refuses cookies older than the 5-minute flow TTL, calls MSAL Node's `acquireTokenByCode` with the stored verifier, validates `amr` is non-empty (the BFF sanity-check per ADR-0011 — Entra Conditional Access on the org side does the real enforcement), extracts `oid` / `tid` / `preferred_username` / `name` / `amr` into an `AuthenticatedUser` shape.
- **[`auth.cookie.ts`](apps/portal-bff/src/auth/auth.cookie.ts)** gains `clearPreAuthCookieOptions()` mirroring the set-options minus `maxAge` so the browser actually drops the cookie. (Cookies match by name + path + secure; getting any of those wrong leaves the old cookie in place.)
- **[`AuthController.callback()`](apps/portal-bff/src/auth/auth.controller.ts)** — `@Get('callback')`. Always clears the cookie first (single-use). Bails on Entra-side errors (`?error=`), missing query params, missing or malformed cookie — each branch logs a structured Pino warning and redirects with the right `auth_error` code. On `AuthCodeFlowException`, logs + redirects with the typed `kind`. On success, logs an `auth.signed_in` event with `oid`, `tid`, `username`, `amr` (PII-sensitive bits only; no tokens), then 302s to `entra.postLogoutRedirectUri`.

## Decisions worth flagging

- **`postLogoutRedirectUri` reused as the SPA root URL.** Semantically a tiny stretch (its OIDC role is the post-logout destination) but the value is the same. Avoids one more env var until / unless the two URLs need to diverge.
- **Cookie cleared FIRST**, before any branching. Single-use is a property we want guaranteed regardless of which path exits the handler — overlap with a parallel /login from the same browser session would otherwise leak a usable cookie.
- **`auth.signed_in` logged via Pino, not via the audit module.** ADR-0013 wants this in the audit table; pairing audit with the session that ships in the next PR keeps the audit row carrying a `session_id` (otherwise it'd reference a "phantom" auth event with no follow-up).
- **`amr` non-empty is the BFF's check; the Conditional Access policy is what enforces "MFA happened".** ADR-0011 explicitly factors it this way — empty `amr` would indicate a policy misconfiguration where MFA never fired.

## Verification

- `nx run-many -t lint test build --projects=portal-bff` — green.
- **52 / 52 specs** (was 39; +13 across the new completeFlow branches and callback branches).
- Service spec covers happy path + 6 failure modes (state mismatch, flow expired, amr missing, MSAL throws, MSAL returns null, oid claim missing).
- Controller spec covers happy redirect, Entra error, missing cookie, AuthCodeFlowException branch, missing query, malformed cookie.

## Manual smoke test (end-to-end)

1. `apps/portal-bff/.env` carries real `ENTRA_*` + `SESSION_SECRET`.
2. `nx serve portal-bff` and `nx serve portal-shell`.
3. Open `http://localhost:3000/api/auth/login` → redirects to Entra.
4. Authenticate. Entra redirects to `http://localhost:3000/api/auth/callback?code=…&state=…`.
5. BFF processes; redirects to `http://localhost:4200/`. Pino log shows `auth.signed_in` with the user's `oid`, `tid`, `username`, `amr`.
6. Tamper test: open the link again, hand-edit the `state=` in the callback URL → BFF redirects with `?auth_error=state-mismatch`.

## What this PR explicitly does NOT do

- **Persist a session.** The user is "authenticated" from the BFF's point of view (identity resolved + logged) but the next request lands anonymous. Closes in the Redis sessions PR per ADR-0010.
- **Audit log entry.** Pairs with sessions so the row carries a `session_id`.
- **Logout / `/me`.** Land after sessions.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #107

apps/portal-bff/src/auth/auth.errors.ts

@@ -0,0 +1,29 @@
+/**
+ * Typed failure modes of the OIDC callback. The controller maps each
+ * to a query-param identifier on the SPA redirect so the front-end
+ * can render an appropriate message without inventing strings.
+ *
+ * Keep this list flat — the controller's `switch` is exhaustive
+ * thanks to TypeScript's narrowing on `kind`.
+ */
+export type AuthCodeFlowError =
+  | { kind: 'state-mismatch' }
+  | { kind: 'flow-expired' }
+  | { kind: 'amr-missing' }
+  | { kind: 'token-exchange-failed'; cause: string };
+
+export class AuthCodeFlowException extends Error {
+  constructor(readonly failure: AuthCodeFlowError) {
+    super(`auth code flow failed: ${failure.kind}`);
+    this.name = 'AuthCodeFlowException';
+  }
+}
+
+/**
+ * Stable short codes used as `?auth_error=<code>` on the SPA-bound
+ * redirect. Same identifiers as the discriminant `kind` so log
+ * grepping correlates trivially.
+ */
+export function authErrorCode(failure: AuthCodeFlowError): string {
+  return failure.kind;
+}