refactor(backend): collapse entitlement provider abstraction back to LicenseService (#889)

Removes backend/src/entitlements/ (registry, loadProvider,
CommunityEntitlementProvider, types, headers, normalize) and the two
abstraction-only tests. Relocates headers/normalize/types to
services/license-*.ts. Swaps 22 consumer call sites from
getEntitlementProvider() to LicenseService.getInstance(). Drops the
Dockerfile install step plus PRO_PACKAGE_VERSION build-arg and
github_token BuildKit secret in docker-publish.yml. Removes the now
stale no-restricted-imports rule in backend/eslint.config.mjs.

Net: 37 files changed, ~700 lines removed, no behavior change. Local
dev no longer requires GitHub Packages auth to start the backend.

Rationale and revisit conditions in
docs/internal/adrs/2026-05-02-collapse-entitlement-provider.md.

backend/src/services/license-headers.ts

@@ -0,0 +1,9 @@
+/**
+ * HTTP header names used for Distributed License Enforcement between
+ * Sencho instances. A primary instance proxies tier-gated requests to
+ * its remote fleet nodes and asserts the license state via these
+ * headers; the remote node trusts the headers when the request is
+ * authenticated as a node_proxy bearer.
+ */
+export const PROXY_TIER_HEADER = 'x-sencho-tier';
+export const PROXY_VARIANT_HEADER = 'x-sencho-variant';