fix(billing): use tier_id instead of tier_code for feature limit endpoints

Tier codes are not unique across platforms (e.g., "essential" exists for
OMS, marketplace, and loyalty). Using tier_code caused feature limits to
be saved to the wrong tier. Switched to tier_id (unique PK) in routes,
service, and frontend JS. Added comprehensive unit and integration tests
including cross-platform isolation regression tests.

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

app/modules/billing/services/feature_service.py

@@ -450,30 +450,24 @@ class FeatureService:
     # Tier Feature Limit Management
     # =========================================================================
 
-    def get_tier_feature_limits(self, db: Session, tier_code: str) -> list:
+    def get_tier_feature_limits(self, db: Session, tier_id: int) -> list:
         """Get feature limits for a tier."""
-        from app.modules.billing.services import admin_subscription_service
-
-        tier = admin_subscription_service.get_tier_by_code(db, tier_code)
         return (
             db.query(TierFeatureLimit)
-            .filter(TierFeatureLimit.tier_id == tier.id)
+            .filter(TierFeatureLimit.tier_id == tier_id)
             .order_by(TierFeatureLimit.feature_code)
             .all()
         )
 
-    def upsert_tier_feature_limits(self, db: Session, tier_code: str, entries: list[dict]) -> list:
+    def upsert_tier_feature_limits(self, db: Session, tier_id: int, entries: list[dict]) -> list:
         """Replace feature limits for a tier. Returns list of new TierFeatureLimit objects."""
-        from app.modules.billing.services import admin_subscription_service
-
-        tier = admin_subscription_service.get_tier_by_code(db, tier_code)
-        db.query(TierFeatureLimit).filter(TierFeatureLimit.tier_id == tier.id).delete()
+        db.query(TierFeatureLimit).filter(TierFeatureLimit.tier_id == tier_id).delete()
         new_rows = []
         for entry in entries:
             if not entry.get("enabled", True):
                 continue
             row = TierFeatureLimit(
-                tier_id=tier.id,
+                tier_id=tier_id,
                 feature_code=entry["feature_code"],
                 limit_value=entry.get("limit_value"),
             )