Import artdag.nodes to register built-in executors

Without importing artdag.nodes, the EFFECT executor was never
registered, causing "No executor for node type: EFFECT" errors.

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

tasks.py

@@ -22,6 +22,7 @@ from celery_app import app
 from artdag import DAG, Node, NodeType
 from artdag.engine import Engine
 from artdag.executor import register_executor, Executor, get_executor
+import artdag.nodes  # Register all built-in executors (SOURCE, EFFECT, etc.)
 
 # Add effects to path (use env var in Docker, fallback to home dir locally)
 EFFECTS_PATH = Path(os.environ.get("EFFECTS_PATH", str(Path.home() / "artdag-effects")))