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/common/artdag_common/middleware/__init__.py

@@ -0,0 +1,16 @@
+"""
+Middleware and FastAPI dependencies for Art-DAG servers.
+"""
+
+from .auth import UserContext, get_user_from_cookie, get_user_from_header, require_auth
+from .content_negotiation import wants_html, wants_json, ContentType
+
+__all__ = [
+    "UserContext",
+    "get_user_from_cookie",
+    "get_user_from_header",
+    "require_auth",
+    "wants_html",
+    "wants_json",
+    "ContentType",
+]