fix(subscriptions): fix subscription banner not showing on merchant detail page

Fix 3 bugs preventing the subscription banner from rendering on
/admin/merchants/{id}: correct API response parsing for platforms and
tiers endpoints (response.items → response.platforms/response.tiers),
and replace broken model_validator with field_validator for tier code
extraction in the billing schema.

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

app/modules/billing/schemas/billing.py

@@ -7,7 +7,7 @@ Used for admin subscription management and merchant-level billing.
 
 from datetime import datetime
 
-from pydantic import BaseModel, ConfigDict, Field
+from pydantic import BaseModel, ConfigDict, Field, field_validator
 
 
 # ============================================================================
@@ -117,7 +117,7 @@ class MerchantSubscriptionAdminResponse(BaseModel):
     id: int
     merchant_id: int
     platform_id: int
-    tier_id: int | None = None
+    tier: str | None = None
 
     status: str
     is_annual: bool
@@ -137,6 +137,14 @@ class MerchantSubscriptionAdminResponse(BaseModel):
     created_at: datetime
     updated_at: datetime
 
+    @field_validator("tier", mode="before")
+    @classmethod
+    def extract_tier_code(cls, v):
+        """Convert SubscriptionTier ORM object to its code string."""
+        if v is not None and hasattr(v, "code"):
+            return v.code
+        return v
+
 
 class MerchantSubscriptionWithMerchant(MerchantSubscriptionAdminResponse):
     """Subscription response with merchant info."""

app/modules/tenancy/static/admin/js/merchant-detail.js

@@ -102,7 +102,7 @@ function adminMerchantDetail() {
         async loadPlatforms() {
             try {
                 const response = await apiClient.get('/admin/platforms');
-                const platforms = response.items || response;
+                const platforms = response.platforms || [];
                 const oms = platforms.find(p => p.code === 'oms');
                 if (oms) {
                     this.platformId = oms.id;
@@ -155,7 +155,7 @@ function adminMerchantDetail() {
 
             try {
                 const response = await apiClient.get('/admin/subscriptions/tiers');
-                this.tiers = response.items || response;
+                this.tiers = response.tiers || [];
                 merchantDetailLog.info('Loaded tiers:', this.tiers.length);
             } catch (error) {
                 merchantDetailLog.warn('Failed to load tiers:', error.message);