feat(auth): membership authority endpoint + fail-closed API auth (RBAC Phase 1)

GET /v1/users/{id}/memberships answers 'which tenants does this JWT
subject belong to, with which org_roles and entitlements' (ratified
auth design, model B2). Keycloak supplies user->tenant links and roles
via the Adapter (attribute projection until the realm migrates to
Organizations); registered tenants override status/plan/products from
registry tables.

New internal/authn verifies Keycloak bearer tokens (OIDC discovery +
JWKS, audience AUTH_EXPECTED_AUDIENCE, default tenant-registry). With
AUTH_ENABLED=true all routes except /healthz + /readyz require a token
and the server refuses to start if the verifier cannot initialize;
false (dev default) keeps the API open. Completes the M5.2-deferred
org_roles lookup and replaces the 'M4.3 adds JWT validation' TODO in
the spec.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

go.mod

@@ -3,7 +3,9 @@ module gitea.meghsakha.com/platform/tenant-registry
 go 1.25.0
 
 require (
+	github.com/coreos/go-oidc/v3 v3.20.0
 	github.com/getkin/kin-openapi v0.138.0
+	github.com/go-jose/go-jose/v4 v4.1.4
 	github.com/golang-migrate/migrate/v4 v4.19.1
 	github.com/google/uuid v1.6.0
 	github.com/jackc/pgerrcode v0.0.0-20250907135507-afb5586c32a6
@@ -75,6 +77,7 @@ require (
 	go.opentelemetry.io/otel v1.41.0 // indirect
 	go.opentelemetry.io/otel/metric v1.41.0 // indirect
 	go.opentelemetry.io/otel/trace v1.41.0 // indirect
+	golang.org/x/oauth2 v0.36.0 // indirect
 	golang.org/x/sync v0.20.0 // indirect
 	golang.org/x/sys v0.44.0 // indirect
 	golang.org/x/text v0.37.0 // indirect