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/constants.py

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+"""
+Shared constants for Art-DAG servers.
+"""
+
+# CDN URLs
+TAILWIND_CDN = "https://cdn.tailwindcss.com"
+HTMX_CDN = "https://unpkg.com/htmx.org@1.9.10"
+CYTOSCAPE_CDN = "https://cdnjs.cloudflare.com/ajax/libs/cytoscape/3.28.1/cytoscape.min.js"
+DAGRE_CDN = "https://cdnjs.cloudflare.com/ajax/libs/dagre/0.8.5/dagre.min.js"
+CYTOSCAPE_DAGRE_CDN = "https://cdn.jsdelivr.net/npm/cytoscape-dagre@2.5.0/cytoscape-dagre.min.js"
+
+# Node colors for DAG visualization
+NODE_COLORS = {
+    "SOURCE": "#3b82f6",    # Blue - input sources
+    "EFFECT": "#22c55e",    # Green - processing effects
+    "OUTPUT": "#a855f7",    # Purple - final outputs
+    "ANALYSIS": "#f59e0b",  # Amber - analysis nodes
+    "_LIST": "#6366f1",     # Indigo - list aggregation
+    "default": "#6b7280",   # Gray - unknown types
+}
+
+# Status colors
+STATUS_COLORS = {
+    "completed": "bg-green-600",
+    "cached": "bg-blue-600",
+    "running": "bg-yellow-600",
+    "pending": "bg-gray-600",
+    "failed": "bg-red-600",
+}
+
+# Tailwind dark theme configuration
+TAILWIND_CONFIG = """
+<script>
+tailwind.config = {
+    darkMode: 'class',
+    theme: {
+        extend: {
+            colors: {
+                dark: {
+                    600: '#374151',
+                    700: '#1f2937',
+                    800: '#111827',
+                    900: '#030712',
+                }
+            }
+        }
+    }
+}
+</script>
+"""
+
+# Default pagination settings
+DEFAULT_PAGE_SIZE = 20
+MAX_PAGE_SIZE = 100