fix(authelia): use adapter's own client_id/redirect_uri in AuthorizeURL

The adapter was forwarding the downstream client's client_id and
redirect_uri to Authelia, which would always be rejected — Authelia
only recognises client_id=keycape and its registered callback URI.
Also removed downstream PKCE forwarding: KeyCape is a confidential
OIDC client to Authelia and authenticates via client_secret instead.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/internal/adapters/authelia/adapter.go

@@ -37,26 +37,20 @@ func New(cfg Config, httpClient HTTPClient) *AutheliaAdapter {
 
 // AuthorizeURL builds the Authelia OIDC authorization URL to which the user
 // should be redirected.
+//
+// KeyCape is a confidential OIDC client to Authelia. The adapter always uses
+// its own registered client_id and redirect_uri — NOT the downstream client's
+// values — and requests the full fixed scope set. PKCE is omitted because
+// the confidential client_secret authenticates the token exchange instead.
 func (a *AutheliaAdapter) AuthorizeURL(_ context.Context, req domain.AuthRequest) (string, error) {
 	base := strings.TrimRight(a.cfg.BaseURL, "/") + "/api/oidc/authorization"
 
 	q := url.Values{}
-	q.Set("client_id", req.ClientID)
-	q.Set("redirect_uri", req.RedirectURI)
+	q.Set("client_id", a.cfg.ClientID)
+	q.Set("redirect_uri", a.cfg.RedirectURI)
 	q.Set("response_type", "code")
 	q.Set("state", req.State)
-	if req.Nonce != "" {
-		q.Set("nonce", req.Nonce)
-	}
-	if len(req.Scopes) > 0 {
-		q.Set("scope", strings.Join(req.Scopes, " "))
-	} else {
-		q.Set("scope", "openid profile")
-	}
-	if req.PKCEChallenge != "" {
-		q.Set("code_challenge", req.PKCEChallenge)
-		q.Set("code_challenge_method", req.PKCEChallengeMethod)
-	}
+	q.Set("scope", "openid profile email groups")
 
 	return base + "?" + q.Encode(), nil
 }

src/internal/adapters/authelia/adapter_test.go

@@ -75,6 +75,7 @@ func jsonResponse(body string) *http.Response {
 
 func TestAuthorizeURL_ContainsRequiredParams(t *testing.T) {
 	adapter := authelia.New(testConfig(), &mockHTTPClient{})
+	// Downstream client values — must NOT appear in the Authelia URL.
 	req := domain.AuthRequest{
 		ClientID:            "myapp",
 		RedirectURI:         "https://myapp.local/cb",
@@ -90,22 +91,29 @@ func TestAuthorizeURL_ContainsRequiredParams(t *testing.T) {
 		t.Fatalf("unexpected error: %v", err)
 	}
 
-	checks := []string{
-		"client_id=myapp",
+	// Must use adapter's own client_id and redirect_uri, not the downstream client's.
+	required := []string{
+		"client_id=keycape",
 		"redirect_uri=",
 		"response_type=code",
 		"state=state-abc",
-		"nonce=nonce-xyz",
-		"code_challenge=challenge123",
-		"code_challenge_method=S256",
 		"scope=",
 		"openid",
 	}
-	for _, want := range checks {
+	for _, want := range required {
 		if !strings.Contains(u, want) {
 			t.Errorf("AuthorizeURL missing %q in: %s", want, u)
 		}
 	}
+
+	// Downstream client_id must NOT be forwarded to Authelia.
+	if strings.Contains(u, "client_id=myapp") {
+		t.Errorf("AuthorizeURL must not forward downstream client_id to Authelia, got: %s", u)
+	}
+	// PKCE must NOT be forwarded — confidential client uses client_secret instead.
+	if strings.Contains(u, "code_challenge") {
+		t.Errorf("AuthorizeURL must not include PKCE params for confidential client, got: %s", u)
+	}
 }
 
 func TestAuthorizeURL_UsesBaseURL(t *testing.T) {