Initial artdag-common shared library

Shared components for L1 and L2 servers:
- Jinja2 template system with base template and components
- Middleware for auth and content negotiation
- Pydantic models for requests/responses
- Utility functions for pagination, media, formatting
- Constants for Tailwind/HTMX/Cytoscape CDNs

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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",
+]