feat: fully backend-driven OIDC session flow (session cookie, not bearer token)

Replace the hybrid flow (frontend PKCE + POST /auth/session token
exchange, access token in sessionStorage) with a classic backend-driven
BFF: the browser only ever navigates to GET /api/v1/auth/login and is
redirected straight to the IdP; PKCE verifier/state live server-side in
Redis (SessionStoreService); GET /api/v1/auth/callback (now the
registered IdP redirect URI, replacing the frontend's /auth/callback
route, which is deleted) verifies the id_token, JIT-provisions the
user, creates a Redis-backed session, and sets one httpOnly SameSite=Lax
cookie before redirecting into the app. No token material of any kind
ever reaches the browser.

OidcAuthGuard (per-request bearer JWT verification) is replaced by
SessionAuthGuard (cookie -> Redis session lookup) across every
controller that used it. cookie-parser is now wired into main.ts.

Frontend AuthService shrinks to login()/logout()/ensureSessionChecked();
pkce.ts, auth.interceptor.ts, and the callback component/route are all
removed as dead code under this model.

New required env var: APP_BASE_URL (source of truth for the OIDC
redirect_uri and the post-login redirect target).

Verified end-to-end against the real API, Redis, and a mocked IdP:
login redirect shape, callback cookie + redirect, state-replay
rejection, /users/me 401<->200 around the cookie, and logout.

.env.example

@@ -20,3 +20,7 @@ OIDC_CLIENT_ID=travel-planner-web
 # a real value here; supply it only via the deployment host's secret store /
 # the developer's own shell environment.
 OIDC_CLIENT_SECRET=change-me-outside-source-control
+# APP_BASE_URL: the public origin end users load the app from (used to build the
+# OIDC redirect_uri and the post-login redirect target). Must match a redirect
+# URI registered with the IdP client, e.g. https://travel-planner.example.com
+APP_BASE_URL=http://localhost:4200