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/SchedulerService.ts

@@ -1,7 +1,7 @@
 import { CronExpressionParser } from 'cron-parser';
 import { DatabaseService } from './DatabaseService';
 import type { ScheduledTask } from './DatabaseService';
-import { getEntitlementProvider } from '../entitlements/registry';
+import { LicenseService } from './LicenseService';
 import DockerController from './DockerController';
 import { ComposeService } from './ComposeService';
 import { FileSystemService } from './FileSystemService';
@@ -192,7 +192,7 @@ export class SchedulerService {
             }
             await this.maybeRedetectTrivy();
 
-            const ls = getEntitlementProvider();
+            const ls = LicenseService.getInstance();
             const isPaid = ls.getTier() === 'paid';
             const isAdmiral = isPaid && ls.getVariant() === 'admiral';
             if (!isPaid) return;