refactor: migrate Keto authorizer from HTTP to gRPC SDK

Replace the raw HTTP implementation with the official Ory Keto gRPC SDK
(github.com/ory/keto/proto). This uses typed proto messages and
transactional writes instead of manual URL construction and JSON parsing.

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

Author: pitabwire

go.mod

@@ -16,6 +16,7 @@ require (
 	github.com/lmittmann/tint v1.1.3
 	github.com/nats-io/nats.go v1.48.0
 	github.com/nicksnyder/go-i18n/v2 v2.6.1
+	github.com/ory/keto/proto v0.13.0-alpha.0
 	github.com/panjf2000/ants/v2 v2.11.4
 	github.com/pitabwire/natspubsub v0.7.12
 	github.com/pitabwire/util v0.4.0

go.sum

@@ -180,6 +180,8 @@ github.com/opencontainers/go-digest v1.0.0 h1:apOUWs51W5PlhuyGyz9FCeeBIOUDA/6nW8
 github.com/opencontainers/go-digest v1.0.0/go.mod h1:0JzlMkj0TRzQZfJkVvzbP0HBR3IKzErnv2BNG4W4MAM=
 github.com/opencontainers/image-spec v1.1.1 h1:y0fUlFfIZhPF1W537XOLg0/fcx6zcHCJwooC2xJA040=
 github.com/opencontainers/image-spec v1.1.1/go.mod h1:qpqAh3Dmcf36wStyyWU+kCeDgrGnAve2nCC8+7h8Q0M=
+github.com/ory/keto/proto v0.13.0-alpha.0 h1:9ZzjDbaBgriHGVC8fUJKD1pDqQ9nHEFOO3bT971FfBY=
+github.com/ory/keto/proto v0.13.0-alpha.0/go.mod h1:6RagCXA7X1hhFSVjcy13ruIo8Dq/nj4J0mcN92qL+hY=
 github.com/panjf2000/ants/v2 v2.11.4 h1:UJQbtN1jIcI5CYNocTj0fuAUYvsLjPoYi0YuhqV/Y48=
 github.com/panjf2000/ants/v2 v2.11.4/go.mod h1:8u92CYMUc6gyvTIw8Ru7Mt7+/ESnJahz5EVtqfrilek=
 github.com/pitabwire/natspubsub v0.7.12 h1:C2LlsLr9EHH3VtbFV8dZRjPbCjhd6Pq7odOw+f8dSTc=