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/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",
+]