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.

backend/package.json

@@ -28,6 +28,7 @@
     "@nestjs/common": "^11.0.1",
     "@nestjs/core": "^11.0.1",
     "@nestjs/platform-express": "^11.0.1",
+    "cookie-parser": "^1.4.7",
     "ioredis": "^6.0.0",
     "jose": "^5.10.0",
     "kysely": "0.28.17",
@@ -42,6 +43,7 @@
     "@nestjs/cli": "^11.0.0",
     "@nestjs/schematics": "^11.0.0",
     "@nestjs/testing": "^11.0.1",
+    "@types/cookie-parser": "^1.4.10",
     "@types/express": "^5.0.0",
     "@types/jest": "^30.0.0",
     "@types/node": "^24.0.0",