fix(subscriptions): fix subscription banner not showing on merchant detail page

Fix 3 bugs preventing the subscription banner from rendering on
/admin/merchants/{id}: correct API response parsing for platforms and
tiers endpoints (response.items → response.platforms/response.tiers),
and replace broken model_validator with field_validator for tier code
extraction in the billing schema.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/tenancy/static/admin/js/merchant-detail.js

@@ -102,7 +102,7 @@ function adminMerchantDetail() {
         async loadPlatforms() {
             try {
                 const response = await apiClient.get('/admin/platforms');
-                const platforms = response.items || response;
+                const platforms = response.platforms || [];
                 const oms = platforms.find(p => p.code === 'oms');
                 if (oms) {
                     this.platformId = oms.id;
@@ -155,7 +155,7 @@ function adminMerchantDetail() {
 
             try {
                 const response = await apiClient.get('/admin/subscriptions/tiers');
-                this.tiers = response.items || response;
+                this.tiers = response.tiers || [];
                 merchantDetailLog.info('Loaded tiers:', this.tiers.length);
             } catch (error) {
                 merchantDetailLog.warn('Failed to load tiers:', error.message);