feat: Desk swap / booking transfer

[c0dewhacker]

Summary

Allow two users with confirmed bookings to swap desks for a day, or transfer a booking to a colleague.

Proposed Solution

Booking Transfer

• "Transfer to colleague" button on a confirmed booking
• Search for colleague by name/email → transfer request sent
• Recipient accepts/declines via in-app notification or email link
• Transfer preserves audit trail (original booker + transfer event logged)

Desk Swap

• Two users mutually agree to swap their bookings
• Initiate swap request → other party confirms → both bookings updated atomically
• Advisory lock on both asset IDs during the swap transaction

Priority

🔵 P3 — Low impact, low effort

[c0dewhacker]

Implementation plan (major)

• Transfer: "Transfer to colleague" → request → recipient accepts; reassign booking.userId and write an audit row (original booker + transfer event).
• Swap: two confirmed bookings, mutual confirm, then swap userIds atomically in a transaction that takes the shared asset locks for both assets (lockAssetForBooking in apps/api/src/lib/booking.ts) to stay race-safe.
• Data: BookingTransfer / SwapRequest model + notifications.
• Effort: medium — transactional but contained.