fix(billing): complete billing module — fix tier change, platform support, merchant portal

- Fix admin tier change: resolve tier_code→tier_id in update_subscription(),
  delegate to billing_service.change_tier() for Stripe-connected subs
- Add platform support to admin tiers page: platform column, filter dropdown,
  platform selector in create/edit modal, platform_name in tier API response
- Filter used platforms in create subscription modal on merchant detail page
- Enrich merchant portal API responses with tier code, tier_name, platform_name
- Add eager-load of platform relationship in get_merchant_subscription()
- Remove stale store_name/store_code references from merchant templates
- Add merchant tier change endpoint (POST /change-tier) and tier selector UI
  replacing broken requestUpgrade() button
- Fix subscription detail link to use platform_id instead of sub.id

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

app/modules/billing/static/admin/js/subscription-tiers.js

@@ -22,6 +22,8 @@ function adminSubscriptionTiers() {
         tiers: [],
         stats: null,
         includeInactive: false,
+        platforms: [],
+        filterPlatformId: '',
 
         // Feature management
         features: [],
@@ -51,7 +53,8 @@ function adminSubscriptionTiers() {
             stripe_product_id: '',
             stripe_price_monthly_id: '',
             is_active: true,
-            is_public: true
+            is_public: true,
+            platform_id: null
         },
 
         async init() {
@@ -67,7 +70,8 @@ function adminSubscriptionTiers() {
                 await Promise.all([
                     this.loadTiers(),
                     this.loadStats(),
-                    this.loadFeatures()
+                    this.loadFeatures(),
+                    this.loadPlatforms()
                 ]);
                 tiersLog.info('=== SUBSCRIPTION TIERS PAGE INITIALIZED ===');
             } catch (error) {
@@ -92,6 +96,7 @@ function adminSubscriptionTiers() {
                 params.append('include_inactive', this.includeInactive);
                 if (this.sortBy) params.append('sort_by', this.sortBy);
                 if (this.sortOrder) params.append('sort_order', this.sortOrder);
+                if (this.filterPlatformId) params.append('platform_id', this.filterPlatformId);
 
                 const data = await apiClient.get(`/admin/subscriptions/tiers?${params}`);
                 this.tiers = data.tiers || [];
@@ -125,6 +130,22 @@ function adminSubscriptionTiers() {
             }
         },
 
+        async loadPlatforms() {
+            try {
+                const data = await apiClient.get('/admin/platforms');
+                this.platforms = (data.platforms || []).map(p => ({ id: p.id, name: p.name }));
+                tiersLog.info(`Loaded ${this.platforms.length} platforms`);
+            } catch (error) {
+                tiersLog.error('Failed to load platforms:', error);
+            }
+        },
+
+        getPlatformName(platformId) {
+            if (!platformId) return 'Global';
+            const platform = this.platforms.find(p => p.id === platformId);
+            return platform ? platform.name : `Platform #${platformId}`;
+        },
+
         openCreateModal() {
             this.editingTier = null;
             this.formData = {
@@ -137,7 +158,8 @@ function adminSubscriptionTiers() {
                 stripe_product_id: '',
                 stripe_price_monthly_id: '',
                 is_active: true,
-                is_public: true
+                is_public: true,
+                platform_id: null
             };
             this.showModal = true;
         },
@@ -154,7 +176,8 @@ function adminSubscriptionTiers() {
                 stripe_product_id: tier.stripe_product_id || '',
                 stripe_price_monthly_id: tier.stripe_price_monthly_id || '',
                 is_active: tier.is_active,
-                is_public: tier.is_public
+                is_public: tier.is_public,
+                platform_id: tier.platform_id || null
             };
             this.showModal = true;
         },