Update planning docs for processor-agnostic payment linking

Replace stripe_customer_id on account with account_processor_link
table. Update account_payment_method to use processor enum +
processor_payment_method_id instead of Stripe-specific fields.
Supports multiple simultaneous processors for migration scenarios.

planning/08_Domain_Payments_Billing.md

@@ -10,25 +10,27 @@ The Payments domain describes how the application integrates with Stripe for all
 
 
 
-# 2. Stripe Entity Model
+# 2. Processor Entity Model
 
-Stripe Entity
+Payment processor customer and payment method references are stored in processor-agnostic tables, not directly on the account. This supports running multiple processors simultaneously (e.g. during migration) and adding new processors without schema changes.
+
+Processor Concept
 
 Maps To
 
 Notes
 
-Customer
+Customer (Stripe cus_xxx, GP customer token)
 
-account
+account_processor_link
 
-One Stripe customer per billing account
+One link per processor per account — multiple links allowed during migration
 
-PaymentMethod
+PaymentMethod (Stripe pm_xxx, GP payment token)
 
 account_payment_method
 
-pm_xxx stored as reference only
+Processor-agnostic — `processor` enum + `processor_payment_method_id`
 
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