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/schemas/billing.py

@@ -74,6 +74,7 @@ class SubscriptionTierResponse(BaseModel):
     price_monthly_cents: int
     price_annual_cents: int | None = None
     platform_id: int | None = None
+    platform_name: str | None = None
     stripe_product_id: str | None = None
     stripe_price_monthly_id: str | None = None
     stripe_price_annual_id: str | None = None