fix(subscriptions): fix subscription UI and API after store→merchant migration

Store detail page now shows all platform subscriptions instead of always
"No Subscription Found". Subscriptions listing page renamed from Store
to Merchant throughout (template, JS, menu, i18n) with Platform column
added. Tiers API supports platform_id filtering.

Merchant detail page no longer hardcodes 'oms' platform — loads all
platforms, shows subscription cards per platform with labels, and the
Create Subscription modal includes a platform selector with
platform-filtered tiers. Create button always accessible in Quick Actions.

Edit modal on /admin/subscriptions loads tiers from API filtered by
platform instead of hardcoded options, sends tier_code (not tier) to
match PATCH schema, and shows platform context.

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

app/modules/billing/services/admin_subscription_service.py

@@ -40,14 +40,20 @@ class AdminSubscriptionService:
     # =========================================================================
 
     def get_tiers(
-        self, db: Session, include_inactive: bool = False
+        self, db: Session, include_inactive: bool = False, platform_id: int | None = None
     ) -> list[SubscriptionTier]:
-        """Get all subscription tiers."""
+        """Get all subscription tiers, optionally filtered by platform."""
         query = db.query(SubscriptionTier)
 
         if not include_inactive:
             query = query.filter(SubscriptionTier.is_active == True)  # noqa: E712
 
+        if platform_id is not None:
+            query = query.filter(
+                (SubscriptionTier.platform_id == platform_id)
+                | (SubscriptionTier.platform_id.is_(None))
+            )
+
         return query.order_by(SubscriptionTier.display_order).all()
 
     def get_tier_by_code(self, db: Session, tier_code: str) -> SubscriptionTier: