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/models/__init__.py

@@ -0,0 +1,25 @@
+"""
+Shared Pydantic models for Art-DAG servers.
+"""
+
+from .requests import (
+    PaginationParams,
+    PublishRequest,
+    StorageConfigRequest,
+    MetadataUpdateRequest,
+)
+from .responses import (
+    PaginatedResponse,
+    ErrorResponse,
+    SuccessResponse,
+)
+
+__all__ = [
+    "PaginationParams",
+    "PublishRequest",
+    "StorageConfigRequest",
+    "MetadataUpdateRequest",
+    "PaginatedResponse",
+    "ErrorResponse",
+    "SuccessResponse",
+]