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/auth/router.py

@@ -0,0 +1,3 @@
+# Re-export from original router.
+from app.api.routers.auth import router
+__all__ = ["router"]