fix(subscriptions): fix subscription UI and API after store→merchant migration

Store detail page now shows all platform subscriptions instead of always
"No Subscription Found". Subscriptions listing page renamed from Store
to Merchant throughout (template, JS, menu, i18n) with Platform column
added. Tiers API supports platform_id filtering.

Merchant detail page no longer hardcodes 'oms' platform — loads all
platforms, shows subscription cards per platform with labels, and the
Create Subscription modal includes a platform selector with
platform-filtered tiers. Create button always accessible in Quick Actions.

Edit modal on /admin/subscriptions loads tiers from API filtered by
platform instead of hardcoded options, sends tier_code (not tier) to
match PATCH schema, and shows platform context.

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

app/modules/billing/static/admin/js/subscriptions.js

@@ -39,17 +39,21 @@ function adminSubscriptions() {
         },
 
         // Sorting
-        sortBy: 'store_name',
+        sortBy: 'merchant_name',
         sortOrder: 'asc',
 
         // Modal state
         showModal: false,
         editingSub: null,
         formData: {
-            tier: '',
+            tier_code: '',
             status: ''
         },
 
+        // Tiers for edit modal
+        editTiers: [],
+        loadingTiers: false,
+
         // Feature overrides
         featureOverrides: [],
         quantitativeFeatures: [],
@@ -208,15 +212,34 @@ function adminSubscriptions() {
         async openEditModal(sub) {
             this.editingSub = sub;
             this.formData = {
-                tier: sub.tier,
+                tier_code: sub.tier,
                 status: sub.status
             };
             this.featureOverrides = [];
             this.quantitativeFeatures = [];
             this.showModal = true;
 
-            // Load feature catalog and merchant overrides
-            await this.loadFeatureOverrides(sub.merchant_id);
+            // Load tiers filtered by platform and feature overrides in parallel
+            await Promise.all([
+                this.loadEditTiers(sub.platform_id),
+                this.loadFeatureOverrides(sub.merchant_id),
+            ]);
+        },
+
+        async loadEditTiers(platformId) {
+            this.loadingTiers = true;
+            try {
+                const url = platformId
+                    ? `/admin/subscriptions/tiers?platform_id=${platformId}`
+                    : '/admin/subscriptions/tiers';
+                const response = await apiClient.get(url);
+                this.editTiers = response.tiers || [];
+                subsLog.info('Loaded tiers for edit modal:', this.editTiers.length);
+            } catch (error) {
+                subsLog.error('Failed to load tiers:', error);
+            } finally {
+                this.loadingTiers = false;
+            }
         },
 
         closeModal() {
@@ -312,7 +335,7 @@ function adminSubscriptions() {
                 // Save feature overrides
                 await this.saveFeatureOverrides(this.editingSub.merchant_id);
 
-                this.successMessage = `Subscription for "${this.editingSub.store_name || this.editingSub.merchant_name}" updated`;
+                this.successMessage = `Subscription for "${this.editingSub.merchant_name}" updated`;
 
                 this.closeModal();
                 await this.loadSubscriptions();