refactor(entitlements): migrate type-only consumers to entitlements/types (#879)

Follows the Phase 1 EntitlementProvider abstraction. Two files
imported tier types from services/LicenseService via the back-compat
re-export added in Phase 1; this PR points them at the canonical
location at entitlements/types and drops the re-export block.

Migrated:
  - backend/src/types/express.ts
  - backend/src/routes/dashboard.ts

After this PR, services/LicenseService.ts has no public type re-
exports. The remaining imports of services/LicenseService are:
  - entitlements/loadProvider.ts: runtime import of the
    LicenseService class itself, the intentional Phase 1 binding
    site.
  - __tests__/license-service-id-validation.test.ts: imports
    SENCHO_LS_* catalog constants and resolveSenchoVariantFromMeta;
    these are LemonSqueezy-implementation-specific and stay in
    services/LicenseService until Phase 2 moves the file to
    @studio-saelix/sencho-pro.

Phase 2's deletion of services/LicenseService.ts now requires zero
public-core consumer changes outside the loader and the LS-specific
test file.

Test results: 89/89 backend test files clean, 1657 passing tests, 5
pre-existing skips, plus the same pre-existing database-metrics
stress test flake under parallel load that consistently passes solo.

backend/src/routes/dashboard.ts

@@ -3,7 +3,7 @@ import { DatabaseService } from '../services/DatabaseService';
 import { CloudBackupService } from '../services/CloudBackupService';
 import { authMiddleware } from '../middleware/auth';
 import { effectiveTier, effectiveVariant } from '../middleware/tierGates';
-import type { LicenseTier, LicenseVariant } from '../services/LicenseService';
+import type { LicenseTier, LicenseVariant } from '../entitlements/types';
 
 export const dashboardRouter = Router();