feat: authentication with local accounts, sessions, CSRF, RBAC, and OIDC PKCE (phase 3)

- argon2id (t=3, m=64MiB, p=2) PHC hashing honoring embedded params
- token-bucket rate limiting (10/15min per IP+email) on login/register
- opaque 32B session tokens in Mongo, 30-day sliding expiry, logout-all
- CSRF double-submit cookie/header on authenticated mutations
- bootstrap admin from env with forced first-login password change
- requireAuth/requireRole middleware with disabled-account enforcement
- OIDC code flow with PKCE: lazy discovery, account linking only on
  verified email, auto-created developer accounts
- unit tests (RBAC matrix, CSRF, password, rate limiter) + integration
  suite covering the full auth matrix incl. an in-test fake IdP

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docker-compose.yml

@@ -2,6 +2,8 @@ services:
   mongo:
     image: mongo:7
     restart: unless-stopped
+    # modest cache so the whole stack fits comfortably on small hosts
+    command: ["mongod", "--wiredTigerCacheSizeGB", "0.5"]
     # Mongo is reachable only on the compose network; never published to the
     # host (§12).
     volumes: