fix(payment): remove purchase_subscription fields replaced by payment system

The built-in payment system replaces the old external purchase subscription
iframe approach. Remove purchase_subscription_enabled/url from admin settings
interface and form defaults, as the Payment tab now handles this functionality.
Kept in stores/app.ts fallback to match backend DTO response structure.

frontend/src/api/admin/settings.ts

@@ -38,8 +38,6 @@ export interface SystemSettings {
   doc_url: string
   home_content: string
   hide_ccs_import_button: boolean
-  purchase_subscription_enabled: boolean
-  purchase_subscription_url: string
   backend_mode_enabled: boolean
   custom_menu_items: CustomMenuItem[]
   custom_endpoints: CustomEndpoint[]
@@ -156,8 +154,6 @@ export interface UpdateSettingsRequest {
   doc_url?: string
   home_content?: string
   hide_ccs_import_button?: boolean
-  purchase_subscription_enabled?: boolean
-  purchase_subscription_url?: string
   backend_mode_enabled?: boolean
   custom_menu_items?: CustomMenuItem[]
   custom_endpoints?: CustomEndpoint[]

frontend/src/views/admin/SettingsView.vue

@@ -2613,8 +2613,6 @@ const form = reactive<SettingsForm>({
   backend_mode_enabled: false,
   hide_ccs_import_button: false,
   payment_enabled: false,  payment_min_amount: 1,  payment_max_amount: 10000,  payment_daily_limit: 50000,  payment_max_pending_orders: 3,  payment_order_timeout_minutes: 30,  payment_balance_disabled: false,  payment_enabled_types: [],  payment_help_image_url: '',  payment_help_text: '',  payment_product_name_prefix: '',  payment_product_name_suffix: '',  payment_load_balance_strategy: 'round-robin',  payment_cancel_rate_limit_enabled: false,  payment_cancel_rate_limit_max: 10,  payment_cancel_rate_limit_window: 1,  payment_cancel_rate_limit_unit: 'day',  payment_cancel_rate_limit_window_mode: 'rolling',
-  purchase_subscription_enabled: false,
-  purchase_subscription_url: '',
   custom_menu_items: [] as Array<{id: string; label: string; icon_svg: string; url: string; visibility: 'user' | 'admin'; sort_order: number}>,
   custom_endpoints: [] as Array<{name: string; endpoint: string; description: string}>,
   frontend_url: '',