RBAC Phase 1: membership authority endpoint + fail-closed API auth (#14)

Implements the model-B2 membership authority (ratified compliance auth design) and inbound token verification.

- GET /v1/users/{id}/memberships: Keycloak supplies user->tenant links + org_roles (attribute projection until the realm migrates to Organizations); registered tenants override status/plan/products from registry tables (source: registry|keycloak). Closes the M5.2-deferred org_roles stub.
- New internal/authn: OIDC discovery + JWKS verification (go-oidc/v3), audience tenant-registry. /healthz + /readyz stay PUBLIC_EXPLICIT; all other routes INTERNAL_SERVICE_ONLY. AUTH_ENABLED=true refuses to start on incomplete config (fail-closed); false (dev default) keeps current behavior.
- Full suite green incl. postgres testcontainers; openapi.yaml updated (contract test passes).

Activation is a separate step (Auth-5): requires the orca-infra env merge and a portal service token (portal currently calls with no auth).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Sharang Parnerkar <30073382+mighty840@users.noreply.github.com>
Reviewed-on: #14

internal/config/config.go

@@ -20,6 +20,13 @@ type Config struct {
 	KeycloakClientID     string
 	KeycloakClientSecret string
 	KeycloakTimeout      time.Duration
+
+	// Inbound API auth (RBAC Phase 1). With AuthEnabled the server refuses
+	// to start unless OIDC discovery against KeycloakIssuer succeeds, and
+	// every non-health route requires a bearer token whose audience
+	// contains AuthAudience.
+	AuthEnabled  bool
+	AuthAudience string
 }
 
 func Load() (*Config, error) {
@@ -39,6 +46,9 @@ func Load() (*Config, error) {
 		KeycloakClientID:     os.Getenv("KEYCLOAK_CLIENT_ID"),
 		KeycloakClientSecret: os.Getenv("KEYCLOAK_CLIENT_SECRET"),
 		KeycloakTimeout:      10 * time.Second,
+
+		AuthEnabled:  getenv("AUTH_ENABLED", "false") == "true",
+		AuthAudience: getenv("AUTH_EXPECTED_AUDIENCE", "tenant-registry"),
 	}, nil
 }