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/routes/api/admin_features.py

@@ -86,11 +86,11 @@ def get_feature_catalog(
 
 
 @admin_features_router.get(
-    "/tiers/{tier_code}/limits",
+    "/tiers/{tier_id}/limits",
     response_model=list[TierFeatureLimitEntry],
 )
 def get_tier_feature_limits(
-    tier_code: str = Path(..., description="Tier code"),
+    tier_id: int = Path(..., description="Tier ID"),
     current_user: UserContext = Depends(get_current_admin_api),
     db: Session = Depends(get_db),
 ):
@@ -100,7 +100,7 @@ def get_tier_feature_limits(
     Returns all TierFeatureLimit rows associated with the tier,
     each containing a feature_code and its optional limit_value.
     """
-    rows = feature_service.get_tier_feature_limits(db, tier_code)
+    rows = feature_service.get_tier_feature_limits(db, tier_id)
 
     return [
         TierFeatureLimitEntry(
@@ -113,12 +113,12 @@ def get_tier_feature_limits(
 
 
 @admin_features_router.put(
-    "/tiers/{tier_code}/limits",
+    "/tiers/{tier_id}/limits",
     response_model=list[TierFeatureLimitEntry],
 )
 def upsert_tier_feature_limits(
     entries: list[TierFeatureLimitEntry],
-    tier_code: str = Path(..., description="Tier code"),
+    tier_id: int = Path(..., description="Tier ID"),
     current_user: UserContext = Depends(get_current_admin_api),
     db: Session = Depends(get_db),
 ):
@@ -136,15 +136,15 @@ def upsert_tier_feature_limits(
         raise InvalidFeatureCodesError(invalid_codes)
 
     new_rows = feature_service.upsert_tier_feature_limits(
-        db, tier_code, [e.model_dump() for e in entries]
+        db, tier_id, [e.model_dump() for e in entries]
     )
 
     db.commit()
 
     logger.info(
-        "Admin %s replaced tier '%s' feature limits (%d entries)",
+        "Admin %s replaced tier %d feature limits (%d entries)",
         current_user.id,
-        tier_code,
+        tier_id,
         len(new_rows),
     )