feat(billing): migrate frontend templates to feature provider system

Replace hardcoded subscription fields (orders_limit, products_limit,
team_members_limit) across 5 frontend pages with dynamic feature
provider APIs. Add admin convenience endpoint for store subscription
lookup. Remove legacy stubs (StoreSubscription, FeatureCode, Feature,
TIER_LIMITS, FeatureInfo, FeatureUpgradeInfo) and schema aliases.

Pages updated:
- Admin subscriptions: dynamic feature overrides editor
- Admin tiers: correct feature catalog/limits API URLs
- Store billing: usage metrics from /store/billing/usage
- Merchant subscription detail: tier.feature_limits rendering
- Admin store detail: new GET /admin/subscriptions/store/{id} endpoint

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

app/modules/billing/models/__init__.py

@@ -7,40 +7,31 @@ discovered and registered with SQLAlchemy's Base.metadata at startup.
 
 Usage:
     from app.modules.billing.models import (
-        VendorSubscription,
+        MerchantSubscription,
         SubscriptionTier,
         SubscriptionStatus,
         TierCode,
-        Feature,
-        FeatureCode,
+        TierFeatureLimit,
+        MerchantFeatureOverride,
     )
 """
 
+from app.modules.billing.models.merchant_subscription import MerchantSubscription
 from app.modules.billing.models.subscription import (
-    # Enums
-    TierCode,
-    SubscriptionStatus,
     AddOnCategory,
-    BillingPeriod,
-    # Models
-    SubscriptionTier,
     AddOnProduct,
-    VendorAddOn,
-    StripeWebhookEvent,
     BillingHistory,
-    VendorSubscription,
+    BillingPeriod,
     CapacitySnapshot,
-    # Legacy constants
-    TIER_LIMITS,
+    StoreAddOn,
+    StripeWebhookEvent,
+    SubscriptionStatus,
+    SubscriptionTier,
+    TierCode,
 )
-from app.modules.billing.models.feature import (
-    # Enums
-    FeatureCategory,
-    FeatureUILocation,
-    # Model
-    Feature,
-    # Constants
-    FeatureCode,
+from app.modules.billing.models.tier_feature_limit import (
+    MerchantFeatureOverride,
+    TierFeatureLimit,
 )
 
 __all__ = [
@@ -52,18 +43,13 @@ __all__ = [
     # Subscription Models
     "SubscriptionTier",
     "AddOnProduct",
-    "VendorAddOn",
+    "StoreAddOn",
     "StripeWebhookEvent",
     "BillingHistory",
-    "VendorSubscription",
     "CapacitySnapshot",
-    # Legacy constants
-    "TIER_LIMITS",
-    # Feature Enums
-    "FeatureCategory",
-    "FeatureUILocation",
-    # Feature Model
-    "Feature",
-    # Feature Constants
-    "FeatureCode",
+    # Merchant Subscription
+    "MerchantSubscription",
+    # Feature Limits
+    "TierFeatureLimit",
+    "MerchantFeatureOverride",
 ]