fix(billing): complete billing module — fix tier change, platform support, merchant portal

- Fix admin tier change: resolve tier_code→tier_id in update_subscription(),
  delegate to billing_service.change_tier() for Stripe-connected subs
- Add platform support to admin tiers page: platform column, filter dropdown,
  platform selector in create/edit modal, platform_name in tier API response
- Filter used platforms in create subscription modal on merchant detail page
- Enrich merchant portal API responses with tier code, tier_name, platform_name
- Add eager-load of platform relationship in get_merchant_subscription()
- Remove stale store_name/store_code references from merchant templates
- Add merchant tier change endpoint (POST /change-tier) and tier selector UI
  replacing broken requestUpgrade() button
- Fix subscription detail link to use platform_id instead of sub.id

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

app/modules/billing/routes/api/admin.py

@@ -59,8 +59,17 @@ def list_subscription_tiers(
     """List all subscription tiers."""
     tiers = admin_subscription_service.get_tiers(db, include_inactive=include_inactive, platform_id=platform_id)
 
+    from app.modules.tenancy.models import Platform
+
+    platforms_map = {p.id: p.name for p in db.query(Platform).all()}
+    tiers_response = []
+    for t in tiers:
+        resp = SubscriptionTierResponse.model_validate(t)
+        resp.platform_name = platforms_map.get(t.platform_id) if t.platform_id else None
+        tiers_response.append(resp)
+
     return SubscriptionTierListResponse(
-        tiers=[SubscriptionTierResponse.model_validate(t) for t in tiers],
+        tiers=tiers_response,
         total=len(tiers),
     )
 

app/modules/billing/routes/api/merchant.py

@@ -19,6 +19,7 @@ registration under /api/v1/merchants/billing/*).
 import logging
 
 from fastapi import APIRouter, Depends, HTTPException, Path, Query, Request
+from pydantic import BaseModel
 from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_merchant_from_cookie_or_header
@@ -97,13 +98,15 @@ def list_merchant_subscriptions(
     merchant = _get_user_merchant(db, current_user)
     subscriptions = subscription_service.get_merchant_subscriptions(db, merchant.id)
 
-    return {
-        "subscriptions": [
-            MerchantSubscriptionResponse.model_validate(sub)
-            for sub in subscriptions
-        ],
-        "total": len(subscriptions),
-    }
+    items = []
+    for sub in subscriptions:
+        data = MerchantSubscriptionResponse.model_validate(sub).model_dump()
+        data["tier"] = sub.tier.code if sub.tier else None
+        data["tier_name"] = sub.tier.name if sub.tier else None
+        data["platform_name"] = sub.platform.name if sub.platform else ""
+        items.append(data)
+
+    return {"subscriptions": items, "total": len(items)}
 
 
 @router.get("/subscriptions/{platform_id}")
@@ -129,6 +132,11 @@ def get_merchant_subscription(
             detail=f"No subscription found for platform {platform_id}",
         )
 
+    sub_data = MerchantSubscriptionResponse.model_validate(subscription).model_dump()
+    sub_data["tier"] = subscription.tier.code if subscription.tier else None
+    sub_data["tier_name"] = subscription.tier.name if subscription.tier else None
+    sub_data["platform_name"] = subscription.platform.name if subscription.platform else ""
+
     tier_info = None
     if subscription.tier:
         tier = subscription.tier
@@ -142,7 +150,7 @@ def get_merchant_subscription(
         )
 
     return {
-        "subscription": MerchantSubscriptionResponse.model_validate(subscription),
+        "subscription": sub_data,
         "tier": tier_info,
     }
 
@@ -180,6 +188,40 @@ def get_available_tiers(
     }
 
 
+class ChangeTierRequest(BaseModel):
+    """Request for changing subscription tier."""
+
+    tier_code: str
+    is_annual: bool = False
+
+
+@router.post("/subscriptions/{platform_id}/change-tier")
+def change_subscription_tier(
+    request: Request,
+    tier_data: ChangeTierRequest,
+    platform_id: int = Path(..., description="Platform ID"),
+    current_user: UserContext = Depends(get_current_merchant_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Change the subscription tier for a specific platform.
+
+    Handles both Stripe-connected and non-Stripe subscriptions.
+    """
+    merchant = _get_user_merchant(db, current_user)
+    result = billing_service.change_tier(
+        db, merchant.id, platform_id, tier_data.tier_code, tier_data.is_annual
+    )
+    db.commit()
+
+    logger.info(
+        f"Merchant {merchant.id} ({merchant.name}) changed tier to "
+        f"{tier_data.tier_code} on platform={platform_id}"
+    )
+
+    return result
+
+
 @router.post(
     "/subscriptions/{platform_id}/checkout",
     response_model=CheckoutResponse,