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/__init__.py

@@ -0,0 +1,18 @@
+"""
+Art-DAG Common Library
+
+Shared components for L1 (celery) and L2 (activity-pub) servers.
+"""
+
+from .constants import NODE_COLORS, TAILWIND_CDN, HTMX_CDN, CYTOSCAPE_CDN
+from .rendering import create_jinja_env, render, render_fragment
+
+__all__ = [
+    "NODE_COLORS",
+    "TAILWIND_CDN",
+    "HTMX_CDN",
+    "CYTOSCAPE_CDN",
+    "create_jinja_env",
+    "render",
+    "render_fragment",
+]