Add accounts, members, and processor-agnostic payment linking

- account table (billing entity, soft-delete, company-scoped)
- member table (people on an account, is_minor from DOB)
- account_processor_link table (maps accounts to any payment
  processor — stripe, global_payments — instead of stripe_customer_id
  directly on account)
- Full CRUD routes + search (name, email, phone, account_number)
- Member routes nested under accounts with isMinor auto-calculation
- Zod validation schemas in @forte/shared
- 19 tests passing

packages/backend/src/db/index.ts

@@ -1,2 +1,3 @@
 export * from './schema/stores.js'
 export * from './schema/users.js'
+export * from './schema/accounts.js'