Incorporate art-dag-mono repo into artdag/ subfolder

Merges full history from art-dag/mono.git into the monorepo
under the artdag/ directory. Contains: core (DAG engine),
l1 (Celery rendering server), l2 (ActivityPub registry),
common (shared templates/middleware), client (CLI), test (e2e).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

git-subtree-dir: artdag
git-subtree-mainline: 1a179de547
git-subtree-split: 4c2e716558

artdag/l1/app/services/__init__.py

@@ -0,0 +1,15 @@
+"""
+L1 Server Services.
+
+Business logic layer between routers and repositories.
+"""
+
+from .run_service import RunService
+from .recipe_service import RecipeService
+from .cache_service import CacheService
+
+__all__ = [
+    "RunService",
+    "RecipeService",
+    "CacheService",
+]