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

@@ -156,11 +156,16 @@ function adminMerchantDetail() {
             }
         },
 
-        // Open create subscription modal
+        // Open create subscription modal (only show platforms without existing subscriptions)
         async openCreateSubscriptionModal() {
-            const firstPlatformId = this.platforms.length > 0 ? this.platforms[0].id : null;
-            this.createForm = { platform_id: firstPlatformId, tier_code: 'essential', status: 'trial', trial_days: 14, is_annual: false };
-            await this.loadTiers(firstPlatformId);
+            const usedPlatformIds = this.subscriptions.map(e => e.platform_id);
+            const available = this.platforms.filter(p => !usedPlatformIds.includes(p.id));
+            if (available.length === 0) {
+                Utils.showToast('All platforms already have subscriptions', 'info');
+                return;
+            }
+            this.createForm = { platform_id: available[0].id, tier_code: 'essential', status: 'trial', trial_days: 14, is_annual: false };
+            await this.loadTiers(available[0].id);
             this.showCreateSubscriptionModal = true;
         },