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/admin_subscription_service.py

@@ -286,6 +286,70 @@ class AdminSubscriptionService:
         p = db.query(Platform).filter(Platform.id == platform_id).first()
         return p.name if p else None
 
+    # =========================================================================
+    # Merchant Subscriptions with Usage
+    # =========================================================================
+
+    def get_merchant_subscriptions_with_usage(
+        self, db: Session, merchant_id: int
+    ) -> list[dict]:
+        """Get all subscriptions for a merchant with tier info and feature usage.
+
+        Returns a list of dicts, each containing:
+        - subscription: serialized MerchantSubscription
+        - tier: tier info dict (code, name, feature_codes)
+        - features: list of quantitative usage metrics
+        - platform_id: int
+        - platform_name: str
+        """
+        from app.modules.billing.schemas import MerchantSubscriptionAdminResponse
+        from app.modules.billing.services.feature_service import feature_service
+        from app.modules.billing.services.subscription_service import (
+            subscription_service,
+        )
+
+        subs = subscription_service.get_merchant_subscriptions(db, merchant_id)
+        platforms_map = self.get_platform_names_map(db)
+
+        results = []
+        for sub in subs:
+            features_summary = feature_service.get_merchant_features_summary(
+                db, merchant_id, sub.platform_id
+            )
+
+            tier_info = None
+            if sub.tier:
+                tier_info = {
+                    "code": sub.tier.code,
+                    "name": sub.tier.name,
+                    "feature_codes": [
+                        fl.feature_code for fl in (sub.tier.feature_limits or [])
+                    ],
+                }
+
+            usage_metrics = []
+            for fs in features_summary:
+                if fs.feature_type == "quantitative" and fs.enabled:
+                    usage_metrics.append({
+                        "name": fs.name_key.replace("_", " ").title(),
+                        "current": fs.current or 0,
+                        "limit": fs.limit,
+                        "percentage": fs.percent_used or 0,
+                        "is_unlimited": fs.limit is None,
+                        "is_at_limit": fs.remaining == 0 if fs.remaining is not None else False,
+                        "is_approaching_limit": (fs.percent_used or 0) >= 80,
+                    })
+
+            results.append({
+                "subscription": MerchantSubscriptionAdminResponse.model_validate(sub).model_dump(),
+                "tier": tier_info,
+                "features": usage_metrics,
+                "platform_id": sub.platform_id,
+                "platform_name": platforms_map.get(sub.platform_id, ""),
+            })
+
+        return results
+
     # =========================================================================
     # Statistics
     # =========================================================================