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/services/subscription_service.py

@@ -96,7 +96,8 @@ class SubscriptionService:
             db.query(MerchantSubscription)
             .options(
                 joinedload(MerchantSubscription.tier)
-                .joinedload(SubscriptionTier.feature_limits)
+                .joinedload(SubscriptionTier.feature_limits),
+                joinedload(MerchantSubscription.platform),
             )
             .filter(
                 MerchantSubscription.merchant_id == merchant_id,