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/conftest.py

@@ -0,0 +1,93 @@
+"""
+Pytest fixtures for art-celery tests.
+"""
+
+import pytest
+from typing import Any, Dict, List
+
+
+@pytest.fixture
+def sample_compiled_nodes() -> List[Dict[str, Any]]:
+    """Sample nodes as produced by the S-expression compiler."""
+    return [
+        {
+            "id": "source_1",
+            "type": "SOURCE",
+            "config": {"asset": "cat"},
+            "inputs": [],
+            "name": None,
+        },
+        {
+            "id": "source_2",
+            "type": "SOURCE",
+            "config": {
+                "input": True,
+                "name": "Second Video",
+                "description": "A user-provided video",
+            },
+            "inputs": [],
+            "name": "second-video",
+        },
+        {
+            "id": "effect_1",
+            "type": "EFFECT",
+            "config": {"effect": "identity"},
+            "inputs": ["source_1"],
+            "name": None,
+        },
+        {
+            "id": "effect_2",
+            "type": "EFFECT",
+            "config": {"effect": "invert", "intensity": 1.0},
+            "inputs": ["source_2"],
+            "name": None,
+        },
+        {
+            "id": "sequence_1",
+            "type": "SEQUENCE",
+            "config": {},
+            "inputs": ["effect_1", "effect_2"],
+            "name": None,
+        },
+    ]
+
+
+@pytest.fixture
+def sample_registry() -> Dict[str, Dict[str, Any]]:
+    """Sample registry with assets and effects."""
+    return {
+        "assets": {
+            "cat": {
+                "cid": "QmXrj6tSSn1vQXxxEY2Tyoudvt4CeeqR9gGQwSt7WFrhMZ",
+                "url": "https://example.com/cat.jpg",
+            },
+        },
+        "effects": {
+            "identity": {
+                "cid": "QmcWhw6wbHr1GDmorM2KDz8S3yCGTfjuyPR6y8khS2tvko",
+            },
+            "invert": {
+                "cid": "QmPWaW5E5WFrmDjT6w8enqvtJhM8c5jvQu7XN1doHA3Z7J",
+            },
+        },
+    }
+
+
+@pytest.fixture
+def sample_recipe(
+    sample_compiled_nodes: List[Dict[str, Any]],
+    sample_registry: Dict[str, Dict[str, Any]],
+) -> Dict[str, Any]:
+    """Sample compiled recipe."""
+    return {
+        "name": "test-recipe",
+        "version": "1.0",
+        "description": "A test recipe",
+        "owner": "@test@example.com",
+        "registry": sample_registry,
+        "dag": {
+            "nodes": sample_compiled_nodes,
+            "output": "sequence_1",
+        },
+        "recipe_id": "Qmtest123",
+    }