Add testing infrastructure and refactor DAG transformation

Testing setup:
- Add pyproject.toml with mypy and pytest configuration
- Add requirements-dev.txt for development dependencies
- Create tests/ directory with test fixtures
- Add 17 unit tests for DAG transformation pipeline

Type annotations:
- Add app/types.py with TypedDict definitions for node configs
- Add typed helper functions: transform_node, build_input_name_mapping,
  bind_inputs, prepare_dag_for_execution
- Refactor run_recipe to use the new typed helpers

Regression tests for today's bugs:
- test_effect_cid_key_not_effect_hash: Verifies CID uses 'cid' key
- test_source_cid_binding_persists: Verifies bound CIDs in final DAG

Run tests with: pytest tests/ -v

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/__init__.py

@@ -0,0 +1 @@
+# Tests for art-celery