Rename tasks.py to legacy_tasks.py to avoid conflict with tasks/ package

The tasks/ directory for 3-phase execution was shadowing the old tasks.py.
Renamed to legacy_tasks.py and updated all imports.

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

celery_app.py

@@ -14,7 +14,7 @@ app = Celery(
     'art_celery',
     broker=REDIS_URL,
     backend=REDIS_URL,
-    include=['tasks', 'tasks.analyze', 'tasks.execute', 'tasks.orchestrate']
+    include=['legacy_tasks', 'tasks', 'tasks.analyze', 'tasks.execute', 'tasks.orchestrate']
 )
 
 app.conf.update(

render.py

@@ -12,7 +12,7 @@ import argparse
 import json
 import sys
 
-from tasks import render_effect
+from legacy_tasks import render_effect
 
 # Known asset hashes
 ASSETS = {

server.py

@@ -39,7 +39,7 @@ from urllib.parse import urlparse
 import yaml
 
 from celery_app import app as celery_app
-from tasks import render_effect, execute_dag, build_effect_dag
+from legacy_tasks import render_effect, execute_dag, build_effect_dag
 from contextlib import asynccontextmanager
 from cache_manager import L1CacheManager, get_cache_manager
 import database