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/l2/requirements.txt

@@ -0,0 +1,13 @@
+fastapi>=0.109.0
+uvicorn>=0.27.0
+requests>=2.31.0
+httpx>=0.27.0
+cryptography>=42.0.0
+bcrypt>=4.0.0
+python-jose[cryptography]>=3.3.0
+markdown>=3.5.0
+python-multipart>=0.0.6
+itsdangerous>=2.1.0
+asyncpg>=0.29.0
+boto3>=1.34.0
+# common (artdag_common) installed from local dir in Dockerfile