feat(fusion_plating): Office User grants Contact Creation + rack-load Phase 1 models

- group_fp_office_user now implies base.group_partner_manager so every office/
  manager role can create contacts (Technicians excluded). Fixes the live
  "create a company contact, it doesn't show" report (AccessError on save).
- New fp.rack.load + fp.rack.load.line models (multi-rack split at Racking,
  Phase 1) with sequence, ACLs, equal-split math, and tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

fusion_plating/fusion_plating/tests/__init__.py

@@ -11,3 +11,4 @@ from . import test_landing_resolver
 from . import test_team_page
 from . import test_sales_manager_gate
 from . import test_migration_workflow
+from . import test_rack_load