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/routes/api/admin.py

@@ -59,8 +59,17 @@ def list_subscription_tiers(
     """List all subscription tiers."""
     tiers = admin_subscription_service.get_tiers(db, include_inactive=include_inactive, platform_id=platform_id)
 
+    from app.modules.tenancy.models import Platform
+
+    platforms_map = {p.id: p.name for p in db.query(Platform).all()}
+    tiers_response = []
+    for t in tiers:
+        resp = SubscriptionTierResponse.model_validate(t)
+        resp.platform_name = platforms_map.get(t.platform_id) if t.platform_id else None
+        tiers_response.append(resp)
+
     return SubscriptionTierListResponse(
-        tiers=[SubscriptionTierResponse.model_validate(t) for t in tiers],
+        tiers=tiers_response,
         total=len(tiers),
     )