feat: add global customizable labels system
Introduce sphere-scoped labels (name, description, color) that replace
the hardcoded feature request categories and add categorization to
kanban tasks.
- New `sphere_labels` and `entity_labels` tables with migration that
converts existing categories into labels
- CRUD API for labels at sphere level, gated by `manageLabels` permission
- Labels on feature requests and kanban tasks (create, update, display)
- Cross-sphere label injection prevention via sphere ownership validation
- Shared LabelBadge and LabelPicker components
- Admin label management page with color picker and delete confirmation
- Entity label cleanup on cascade delete
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>