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/static/admin/js/subscription-tiers.js

@@ -273,7 +273,7 @@ function adminSubscriptionTiers() {
 
             try {
                 // Load tier's current feature limits
-                const data = await apiClient.get(`/admin/subscriptions/features/tiers/${tier.code}/limits`);
+                const data = await apiClient.get(`/admin/subscriptions/features/tiers/${tier.id}/limits`);
                 // data is TierFeatureLimitEntry[]: [{feature_code, limit_value, enabled}]
                 this.selectedFeatures = [];
                 for (const entry of (data || [])) {
@@ -327,7 +327,7 @@ function adminSubscriptionTiers() {
                 }));
 
                 await apiClient.put(
-                    `/admin/subscriptions/features/tiers/${this.selectedTierForFeatures.code}/limits`,
+                    `/admin/subscriptions/features/tiers/${this.selectedTierForFeatures.id}/limits`,
                     entries
                 );