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/routes/imageUpdates.ts

@@ -6,7 +6,7 @@ import { CacheService } from '../services/CacheService';
 import { ImageUpdateService } from '../services/ImageUpdateService';
 import { FileSystemService } from '../services/FileSystemService';
 import { ComposeService } from '../services/ComposeService';
-import { getEntitlementProvider } from '../entitlements/registry';
+import { LicenseService } from '../services/LicenseService';
 import { NotificationService } from '../services/NotificationService';
 import { enforcePolicyPreDeploy } from '../services/PolicyEnforcement';
 import { authMiddleware } from '../middleware/auth';
@@ -215,7 +215,7 @@ autoUpdateRouter.post('/execute', authMiddleware, async (req: Request, res: Resp
     const imageUpdateService = ImageUpdateService.getInstance();
     const compose = ComposeService.getInstance(req.nodeId);
     const db = DatabaseService.getInstance();
-    const atomic = getEntitlementProvider().getTier() === 'paid';
+    const atomic = LicenseService.getInstance().getTier() === 'paid';
     const results: string[] = [];
 
     for (const stackName of stackNames) {