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/middleware/__init__.py

@@ -0,0 +1,16 @@
+"""
+Middleware and FastAPI dependencies for Art-DAG servers.
+"""
+
+from .auth import UserContext, get_user_from_cookie, get_user_from_header, require_auth
+from .content_negotiation import wants_html, wants_json, ContentType
+
+__all__ = [
+    "UserContext",
+    "get_user_from_cookie",
+    "get_user_from_header",
+    "require_auth",
+    "wants_html",
+    "wants_json",
+    "ContentType",
+]