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/eslint.config.mjs

@@ -25,26 +25,6 @@ export default tseslint.config(
       // New in ESLint 10 recommended - existing patterns, suppress until addressed
       'no-useless-assignment': 'warn',
       'preserve-caught-error': 'warn',
-      // The private @studio-saelix/sencho-pro package is loaded at
-      // runtime via dynamic import in entitlements/loadProvider.ts.
-      // A static import would (a) bundle the package into the public
-      // BSL build via TypeScript's module resolution, defeating the
-      // privacy split, and (b) break in Community-only environments
-      // where the package is not installed. Use loadEntitlementProvider()
-      // and the registry instead.
-      // no-restricted-imports flags ES `import` statements only by
-      // default; dynamic `import()` expressions are not flagged unless
-      // we set `allowDynamicImports: false`. The loader uses
-      // `await import('@studio-saelix/sencho-pro')` and is therefore
-      // not affected without an extra opt-in.
-      'no-restricted-imports': ['error', {
-        patterns: [
-          {
-            group: ['@studio-saelix/sencho-pro', '@studio-saelix/sencho-pro/*'],
-            message: 'Do not statically import the private package. Use loadEntitlementProvider() and getEntitlementProvider() from src/entitlements/.',
-          },
-        ],
-      }],
     },
   },
 )