feat(B2): add tenant model + migrations 035/036 + RLS policies

Migration 035: tenants table with 'Schaeffler' default seed.
Migration 036: tenant_id FK on all tables, RLS policies, backfill.
New domains/tenants/ with CRUD router (admin only).
All domain models extended with tenant_id FK.
core/database.py: get_db_for_tenant with RLS context setter.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -3,6 +3,7 @@
 This file ensures that `from app.models import X` continues to work.
 The canonical definitions live in app/domains/*/models.py.
 """
+from app.domains.tenants.models import Tenant
 from app.domains.auth.models import User
 from app.domains.imports.models import Template
 from app.domains.products.models import CadFile, Product
@@ -16,7 +17,7 @@ from app.domains.materials.models import Material, MaterialAlias
 from app.models.system_setting import SystemSetting
 
 __all__ = [
-    "User", "Template", "CadFile", "Product", "Order", "OrderItem", "OrderLine",
+    "Tenant", "User", "Template", "CadFile", "Product", "Order", "OrderItem", "OrderLine",
     "AuditLog", "PricingTier", "OutputType", "RenderTemplate", "ProductRenderPosition",
     "Material", "MaterialAlias", "SystemSetting",
 ]