feat(fusion_repairs): sale.order smart buttons - repairs + maintenance

On the original purchase sale.order:
- Repairs button (fa-wrench) lists all repair.order records where
  x_fc_original_sale_order_id = this SO
- Maintenance button (fa-calendar-check-o) lists all
  fusion.repair.maintenance.contract records spawned from this SO
- Both auto-hide when count is zero
- Both gated by fusion_repairs.group_fusion_repairs_user

Follows the count + action_view_* + oe_stat_button / statinfo pattern
from fusion_claims/views/sale_order_views.xml line ~1176.

Co-authored-by: Cursor <cursoragent@cursor.com>

fusion_repairs/models/__init__.py

@@ -15,4 +15,5 @@ from . import res_users
 from . import res_config_settings
 from . import technician_task
 from . import repair_order
+from . import sale_order
 from . import intake_service