Create cashier authentication flow

[Nodirbek75]

Work with backend

[patoo0x]

📋 Definition of Done — 💰 200,000 sats

Expected Behavior After Fix:

• Merchant owner can create cashier accounts with limited permissions
• Cashiers authenticate with their own credentials (not the merchant's)
• Cashiers can process payments, view transaction history, and print receipts
• Cashiers cannot access merchant settings, wallet management, or withdrawal functions
• Merchant owner can view which cashier processed each transaction
• Merchant owner can add, remove, and manage cashier accounts
• Session management: cashier sessions can be revoked by the merchant

Deliverables:

• Backend PR for cashier auth + permissions model
• POS app PR for cashier login flow and restricted UI
• Unit tests for auth flow, permission boundaries, and session management
• Screen recording showing: merchant creates cashier → cashier logs in → processes payment → merchant sees cashier attribution → merchant revokes cashier
• Lightning address provided in your first comment on this issue

Review Gate:

• All existing tests pass
• Nick signs off (POS)
• Ben signs off (backend)
• Dread final approval

Payment: 200,000 sats via Lightning on merge.
See CONTRIBUTING.md for full details.