feat: add single endpoint for merchant subscriptions with usage data

Replace N+1 per-platform API calls on merchant detail page with a single
GET /admin/subscriptions/merchants/{id} endpoint. Extract shared
subscription+usage aggregation logic into a reusable service method and
refactor the store endpoint to use it.

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

app/modules/billing/services/subscription_service.py

@@ -146,7 +146,8 @@ class SubscriptionService:
         return (
             db.query(MerchantSubscription)
             .options(
-                joinedload(MerchantSubscription.tier),
+                joinedload(MerchantSubscription.tier)
+                .joinedload(SubscriptionTier.feature_limits),
                 joinedload(MerchantSubscription.platform),
             )
             .filter(MerchantSubscription.merchant_id == merchant_id)