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/services/__init__.py

@@ -32,9 +32,6 @@ from app.modules.billing.exceptions import (
 from app.modules.billing.services.feature_service import (
     FeatureService,
     feature_service,
-    FeatureInfo,
-    FeatureUpgradeInfo,
-    FeatureCode,
 )
 from app.modules.billing.services.capacity_forecast_service import (
     CapacityForecastService,
@@ -62,9 +59,6 @@ __all__ = [
     "SubscriptionNotCancelledError",
     "FeatureService",
     "feature_service",
-    "FeatureInfo",
-    "FeatureUpgradeInfo",
-    "FeatureCode",
     "CapacityForecastService",
     "capacity_forecast_service",
     "PlatformPricingService",