Add org-level issue priority definitions that appear in the issue
sidebar. Each priority has a name, color, sort order, and optional
default flag. Follows the same architecture as custom statuses (#502).
Includes:
- IssuePriorityDef model with CRUD operations
- Migration v348 adding issue_priority_def table + priority_id on issues
- Org settings UI for managing priorities
- Issue sidebar dropdown for selecting priority
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>