feat(settings): add default subscriptions for new users

- add default subscriptions to admin settings

- auto-assign subscriptions on register and admin user creation

- add validation/tests and align settings UI with subscription selector patterns

backend/internal/server/api_contract_test.go

@@ -499,6 +499,7 @@ func TestAPIContracts(t *testing.T) {
 					"doc_url": "https://docs.example.com",
 					"default_concurrency": 5,
 					"default_balance": 1.25,
+					"default_subscriptions": [],
 					"enable_model_fallback": false,
 					"fallback_model_anthropic": "claude-3-5-sonnet-20241022",
 					"fallback_model_antigravity": "gemini-2.5-pro",
@@ -620,7 +621,7 @@ func newContractDeps(t *testing.T) *contractDeps {
 	settingRepo := newStubSettingRepo()
 	settingService := service.NewSettingService(settingRepo, cfg)
 
-	adminService := service.NewAdminService(userRepo, groupRepo, &accountRepo, nil, proxyRepo, apiKeyRepo, redeemRepo, nil, nil, nil, nil, nil, nil)
+	adminService := service.NewAdminService(userRepo, groupRepo, &accountRepo, nil, proxyRepo, apiKeyRepo, redeemRepo, nil, nil, nil, nil, nil, nil, nil, nil)
 	authHandler := handler.NewAuthHandler(cfg, nil, userService, settingService, nil, redeemService, nil)
 	apiKeyHandler := handler.NewAPIKeyHandler(apiKeyService)
 	usageHandler := handler.NewUsageHandler(usageService, apiKeyService)