feat(auth): first account on a new server needs the setup token from the server log (M462 #4982)

While no accounts exist, the server mints a random setup token at boot and
logs it. Registering the first account (which becomes admin) must carry it,
so whoever reaches a freshly exposed instance first cannot claim it. The
register page asks GET /api/auth/setup-status and shows a "Setup token"
field in place of the invite field while setup is pending.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/auth_register_test.go

@@ -273,3 +273,75 @@ func TestRegister_FirstAdminRace(t *testing.T) {
 	// not "exactly one" (which would require serializable isolation).
 	t.Logf("admin count after race = %d (>=1 is the invariant)", adminCount)
 }
+
+// With the gate on (as Mount sets it), the first account needs the setup
+// token from the log; a missing or wrong one is refused and creates nothing.
+func TestRegister_FirstUserNeedsSetupToken(t *testing.T) {
+	if os.Getenv("MINSTREL_TEST_DATABASE_URL") == "" {
+		t.Skip("MINSTREL_TEST_DATABASE_URL not set")
+	}
+	h, _ := testHandlers(t)
+	resetUsers(t, h)
+	token, err := auth.NewSetupToken()
+	if err != nil {
+		t.Fatalf("mint: %v", err)
+	}
+	h.setupToken = token
+	h.requireSetupToken = true
+
+	register := func(body string) int {
+		req := httptest.NewRequest(http.MethodPost, "/api/auth/register", bytes.NewReader([]byte(body)))
+		rec := httptest.NewRecorder()
+		h.handleRegister(rec, req)
+		return rec.Code
+	}
+
+	if code := register(`{"username":"squatter","password":"abcd1234"}`); code != http.StatusForbidden {
+		t.Errorf("no token: status = %d, want 403", code)
+	}
+	if code := register(`{"username":"squatter","password":"abcd1234","setup_token":"wrong"}`); code != http.StatusForbidden {
+		t.Errorf("wrong token: status = %d, want 403", code)
+	}
+	var n int
+	if err := h.pool.QueryRow(context.Background(), `SELECT count(*) FROM users`).Scan(&n); err != nil {
+		t.Fatalf("count: %v", err)
+	}
+	if n != 0 {
+		t.Fatalf("a refused registration created %d account(s)", n)
+	}
+
+	if code := register(`{"username":"operator","password":"abcd1234","setup_token":"` + token.Value() + `"}`); code != http.StatusOK {
+		t.Fatalf("right token: status = %d, want 200", code)
+	}
+
+	// Once an account exists the token plays no part: the second user is
+	// governed by registration mode, not the setup token.
+	if _, err := h.pool.Exec(context.Background(),
+		`UPDATE registration_settings SET mode = 'open' WHERE id = true`); err != nil {
+		t.Fatalf("open mode: %v", err)
+	}
+	t.Cleanup(func() {
+		_, _ = h.pool.Exec(context.Background(),
+			`UPDATE registration_settings SET mode = 'invite_only' WHERE id = true`)
+	})
+	if code := register(`{"username":"second","password":"abcd1234"}`); code != http.StatusOK {
+		t.Errorf("second user without token: status = %d, want 200", code)
+	}
+}
+
+func TestSetupToken_MatchesOnlyItself(t *testing.T) {
+	tok, err := auth.NewSetupToken()
+	if err != nil {
+		t.Fatalf("mint: %v", err)
+	}
+	if !tok.Matches(tok.Value()) {
+		t.Error("token does not match itself")
+	}
+	if tok.Matches("") || tok.Matches(tok.Value()+"x") {
+		t.Error("token matched something else")
+	}
+	var none *auth.SetupToken
+	if none.Matches("") || none.Matches("anything") {
+		t.Error("a nil token must fail closed")
+	}
+}