refactor(B1): migrate to domain-driven project structure

Move all models/schemas/services/routers into app/domains/.
Keep backward-compat shims in old locations for imports.
Preserves domains/rendering/tasks.py from Phase A.

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

backend/app/domains/products/router.py

@@ -0,0 +1,7 @@
+# Re-export from original routers (products and cad kept as separate files).
+# The original app/api/routers/products.py and app/api/routers/cad.py are
+# the canonical implementations; they are imported directly in main.py.
+from app.api.routers.products import router as products_router
+from app.api.routers.cad import router as cad_router
+
+__all__ = ["products_router", "cad_router"]