When user A opens an offer, a lock is acquired (locked_by + locked_at).
User B opening the same offer sees a warning banner and the form is
read-only. Lock expires after 5 minutes without heartbeat.
Backend:
- POST /:id/lock — acquire lock (returns 423 if locked by another)
- POST /:id/heartbeat — refresh lock timestamp (every 2 min)
- POST /:id/unlock — release lock
- GET /:id — includes locked_by info
- PUT /:id — auto-releases lock on save
Frontend:
- Acquires lock on page load (edit mode only)
- Sends heartbeat every 2 minutes
- Releases lock on page unmount (navigate away)
- Shows warning banner with locker's name
- All inputs read-only + action buttons hidden when locked
Migration: adds locked_by (INT) and locked_at (DATETIME) to quotations.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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