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/utils/__init__.py

@@ -0,0 +1,19 @@
+"""
+Utility functions shared across Art-DAG servers.
+"""
+
+from .pagination import paginate, get_pagination_params
+from .media import detect_media_type, get_media_extension, is_streamable
+from .formatting import format_date, format_size, truncate_hash, format_duration
+
+__all__ = [
+    "paginate",
+    "get_pagination_params",
+    "detect_media_type",
+    "get_media_extension",
+    "is_streamable",
+    "format_date",
+    "format_size",
+    "truncate_hash",
+    "format_duration",
+]