Bind Nova Poshta API key to cash register (#3)

Each cash register stores its own encrypted NP API key. Status polling uses
register keys and binds an order to the register whose key sees the TTN as
its own (PhoneSender present); ETTN receipts are created from that register.
Migration 0008 moves the old NOVA_POSHTA_API_KEY into the default register.

Closes #3

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/cashRegisters/types.ts

@@ -5,6 +5,8 @@ export interface CashRegister {
   name: string
   fiscal_number: string | null
   license_key_masked: string
+  /** null — ключ Новой Почты не задан: касса не получает посылки. */
+  np_api_key_masked: string | null
   tax_codes: (number | string)[]
   is_active: boolean
   is_default: boolean
@@ -15,6 +17,7 @@ export interface CashRegisterCreate {
   fiscal_number: string | null
   license_key: string
   pin_code: string
+  np_api_key: string | null
   tax_codes: (number | string)[]
   is_default: boolean
 }