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/config/config.go

@@ -66,6 +66,13 @@ type AuthConfig struct {
 	JWKSURL string
 	// RequiredCapability is the permission required for LLM inference (e.g., "llm:inference").
 	RequiredCapability string
+	// AllowListedMachineIdentities admits a caller whose token carries no roles
+	// claim, when its client id is in ZitadelClientIDs. ZITADEL does not assert roles
+	// in machine-user access tokens on this instance, so a service account can never
+	// satisfy RequiredCapability. The allow-list is then the authorisation: the caller
+	// still proves possession of a registered private key, and the entry is a reviewed
+	// line of configuration, not a wildcard.
+	AllowListedMachineIdentities bool
 	// TokenURL is the OIDC token endpoint used for token exchange
 	// (client_credentials / password grants).
 	TokenURL string