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/rendering/router.py

@@ -0,0 +1,5 @@
+# Re-export from original routers.
+from app.api.routers.render_templates import router as render_templates_router
+from app.api.routers.output_types import router as output_types_router
+
+__all__ = ["render_templates_router", "output_types_router"]