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>

internal/http/server_test.go

@@ -25,7 +25,7 @@ func newTestServer(t *testing.T) *Server {
 		t.Fatalf("config: %v", err)
 	}
 	log := slog.New(slog.NewTextHandler(io.Discard, nil))
-	return New(cfg, log, metrics.NewRegistry())
+	return New(cfg, log, metrics.NewRegistry(), nil)
 }
 
 func get(t *testing.T, h http.Handler, path string) *httptest.ResponseRecorder {