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/templates/tenancy/admin/merchant-detail.html

@@ -320,7 +320,7 @@
                 <select x-model="createForm.platform_id"
                         @change="onCreatePlatformChange()"
                         class="w-full px-3 py-2 text-sm border border-gray-300 rounded-lg dark:border-gray-600 dark:bg-gray-700 dark:text-gray-300 focus:outline-none focus:ring-2 focus:ring-purple-500">
-                    <template x-for="p in platforms" :key="p.id">
+                    <template x-for="p in platforms.filter(p => !subscriptions.some(s => s.platform_id === p.id))" :key="p.id">
                         <option :value="p.id" x-text="p.name"></option>
                     </template>
                 </select>