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/main.py

@@ -17,6 +17,7 @@ from app.domains.materials.router import router as materials_router
 from app.domains.rendering.router import render_templates_router, output_types_router
 from app.domains.notifications.router import router as notifications_router
 from app.domains.billing.router import router as pricing_router
+from app.domains.tenants.router import router as tenants_router
 
 
 @asynccontextmanager
@@ -74,6 +75,7 @@ app.include_router(products_router, prefix="/api")
 app.include_router(output_types_router, prefix="/api")
 app.include_router(render_templates_router, prefix="/api")
 app.include_router(notifications_router, prefix="/api")
+app.include_router(tenants_router, prefix="/api")
 
 
 @app.get("/health")