feat(admin-console): Stage 3 — users domain

Add the operator console's user-administration pages over the existing
*user.Service (no new business logic).

- GET  /_gm/users            paginated account list
- GET  /_gm/users/{id}       account detail: profile, entitlement, sanctions
- POST /_gm/users/{id}/block        apply permanent_block (reason required)
- POST /_gm/users/{id}/entitlement  set the entitlement tier
- POST /_gm/users/{id}/soft-delete  soft-delete the account (cascades)

The console depends on a UserAdmin interface (satisfied by *user.Service) so the
pages render in tests without a database. All writes flow through the CSRF
guard, carry the operator as the audit actor, and answer with a 303 redirect;
a generic message page handles not-found, validation, and failure notices.
Unblock is intentionally absent — the admin API exposes no remove-sanction
endpoint.

Tests: list/detail render, not-found, block (with actor/scope/reason
assertions), missing-reason 400, bad-CSRF 403, entitlement, soft-delete
redirect, and the service-unavailable path.

Docs: backend/docs/admin-console.md gains the page inventory.

backend/cmd/backend/main.go

@@ -377,6 +377,7 @@ func run(ctx context.Context) (err error) {
 		CSRF:    consoleCSRF,
 		Monitor: opsstatus.NewStore(db),
 		Ready:   ready,
+		Users:   userSvc,
 		Logger:  logger,
 	})