feat(auth): authorise an allow-listed machine identity without a roles claim

ZITADEL does not put roles into a machine-user access token on this instance. The
grant exists, the scope asks for urn:zitadel:iam:org:project:roles, and the claim
still never arrives; a colon-free role key makes no difference either. A service
account therefore cannot satisfy requiredCapability by any configuration.

auth.allowListedMachineIdentities admits such a caller when its client_id appears in
zitadelClientIds. The caller is still authenticated: it signed an assertion with a
private key whose public half ZITADEL holds. The allow-list entry is a reviewed line
of configuration, not a wildcard.

The new path is off by default, and a token that does state its roles is judged by the
role check as before, so the weaker rule never applies twice.

internal/proxy/machine_identity_test.go

@@ -0,0 +1,65 @@
+package proxy
+
+import (
+	"testing"
+
+	"github.com/golang-jwt/jwt/v5"
+
+	"forgejo.riotpiao.com/rock/homelab-frontend/internal/config"
+)
+
+func handlerWith(allow bool, ids ...string) *Handler {
+	return &Handler{config: &config.Config{Auth: config.AuthConfig{
+		AllowListedMachineIdentities: allow,
+		ZitadelClientIDs:             ids,
+	}}}
+}
+
+func TestAllowListedMachineIdentityAdmitsANamedClientWithoutRoles(t *testing.T) {
+	h := handlerWith(true, "poimen-llm")
+	claims := jwt.MapClaims{"client_id": "poimen-llm", "sub": "397071051344839054"}
+	if !h.allowListedMachineIdentity(claims) {
+		t.Fatalf("an allow-listed machine client must be admitted when the token has no roles")
+	}
+}
+
+func TestAllowListedMachineIdentityRefusesAnUnknownClient(t *testing.T) {
+	h := handlerWith(true, "poimen-llm")
+	claims := jwt.MapClaims{"client_id": "someone-else"}
+	if h.allowListedMachineIdentity(claims) {
+		t.Fatalf("a client that is not on the list must not be admitted")
+	}
+}
+
+func TestAllowListedMachineIdentityRefusesWhenTheTokenStatesRoles(t *testing.T) {
+	h := handlerWith(true, "poimen-llm")
+	claims := jwt.MapClaims{
+		"client_id": "poimen-llm",
+		"urn:zitadel:iam:org:project:roles": map[string]interface{}{"memory:read": nil},
+	}
+	if h.allowListedMachineIdentity(claims) {
+		t.Fatalf("a token with roles must be judged by the role check, not by the allow-list")
+	}
+}
+
+func TestAllowListedMachineIdentityIsOffByDefault(t *testing.T) {
+	h := handlerWith(false, "poimen-llm")
+	claims := jwt.MapClaims{"client_id": "poimen-llm"}
+	if h.allowListedMachineIdentity(claims) {
+		t.Fatalf("without the setting the gateway must fall back to the role check only")
+	}
+}
+
+func TestRolesClaimReadsBothShapes(t *testing.T) {
+	asMap := jwt.MapClaims{"urn:zitadel:iam:org:project:roles": map[string]interface{}{"llm:inference": nil}}
+	if got := rolesClaim(asMap); len(got) != 1 || got[0] != "llm:inference" {
+		t.Fatalf("map shape must yield the role names, got %v", got)
+	}
+	asList := jwt.MapClaims{"roles": []interface{}{"llm:inference", "memory:read"}}
+	if got := rolesClaim(asList); len(got) != 2 {
+		t.Fatalf("list shape must yield every role, got %v", got)
+	}
+	if got := rolesClaim(jwt.MapClaims{}); len(got) != 0 {
+		t.Fatalf("a token with no roles must yield nothing, got %v", got)
+	}
+}