fix: move database queries from API to service layer

Move direct database queries in subscriptions.py to admin_subscription_service:
- Add get_vendor() method to verify vendor exists
- Add get_vendor_usage_counts() method for products/team counts
- Update API endpoints to use service methods
- Remove unused imports (func, Product, VendorUser)

Fixes architecture validation errors [API-002]

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/admin_subscription_service.py

@@ -21,13 +21,14 @@ from app.exceptions import (
     ResourceNotFoundException,
     TierNotFoundException,
 )
+from models.database.product import Product
 from models.database.subscription import (
     BillingHistory,
     SubscriptionStatus,
     SubscriptionTier,
     VendorSubscription,
 )
-from models.database.vendor import Vendor
+from models.database.vendor import Vendor, VendorUser
 
 logger = logging.getLogger(__name__)
 
@@ -197,6 +198,39 @@ class AdminSubscriptionService:
 
         return sub, vendor
 
+    def get_vendor(self, db: Session, vendor_id: int) -> Vendor:
+        """Get a vendor by ID."""
+        vendor = db.query(Vendor).filter(Vendor.id == vendor_id).first()
+
+        if not vendor:
+            raise ResourceNotFoundException("Vendor", str(vendor_id))
+
+        return vendor
+
+    def get_vendor_usage_counts(self, db: Session, vendor_id: int) -> dict:
+        """Get usage counts (products and team members) for a vendor."""
+        products_count = (
+            db.query(func.count(Product.id))
+            .filter(Product.vendor_id == vendor_id)
+            .scalar()
+            or 0
+        )
+
+        team_count = (
+            db.query(func.count(VendorUser.id))
+            .filter(
+                VendorUser.vendor_id == vendor_id,
+                VendorUser.is_active == True,  # noqa: E712
+            )
+            .scalar()
+            or 0
+        )
+
+        return {
+            "products_count": products_count,
+            "team_count": team_count,
+        }
+
     # =========================================================================
     # Billing History
     # =========================================================================