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/content_negotiation.py

@@ -0,0 +1,174 @@
+"""
+Content negotiation utilities.
+
+Helps determine what response format the client wants.
+"""
+
+from enum import Enum
+from typing import Optional
+
+from fastapi import Request
+
+
+class ContentType(Enum):
+    """Response content types."""
+    HTML = "text/html"
+    JSON = "application/json"
+    ACTIVITY_JSON = "application/activity+json"
+    XML = "application/xml"
+
+
+def wants_html(request: Request) -> bool:
+    """
+    Check if the client wants HTML response.
+
+    Returns True if:
+    - Accept header contains text/html
+    - Accept header contains application/xhtml+xml
+    - No Accept header (browser default)
+
+    Args:
+        request: FastAPI request
+
+    Returns:
+        True if HTML is preferred
+    """
+    accept = request.headers.get("accept", "")
+
+    # No accept header usually means browser
+    if not accept:
+        return True
+
+    # Check for HTML preferences
+    if "text/html" in accept:
+        return True
+    if "application/xhtml" in accept:
+        return True
+
+    return False
+
+
+def wants_json(request: Request) -> bool:
+    """
+    Check if the client wants JSON response.
+
+    Returns True if:
+    - Accept header contains application/json
+    - Accept header does NOT contain text/html
+    - Request has .json suffix (convention)
+
+    Args:
+        request: FastAPI request
+
+    Returns:
+        True if JSON is preferred
+    """
+    accept = request.headers.get("accept", "")
+
+    # Explicit JSON preference
+    if "application/json" in accept:
+        # But not if HTML is also requested (browsers often send both)
+        if "text/html" not in accept:
+            return True
+
+    # Check URL suffix convention
+    if request.url.path.endswith(".json"):
+        return True
+
+    return False
+
+
+def wants_activity_json(request: Request) -> bool:
+    """
+    Check if the client wants ActivityPub JSON-LD response.
+
+    Used for federation with other ActivityPub servers.
+
+    Args:
+        request: FastAPI request
+
+    Returns:
+        True if ActivityPub format is preferred
+    """
+    accept = request.headers.get("accept", "")
+
+    if "application/activity+json" in accept:
+        return True
+    if "application/ld+json" in accept:
+        return True
+
+    return False
+
+
+def get_preferred_type(request: Request) -> ContentType:
+    """
+    Determine the preferred content type from Accept header.
+
+    Args:
+        request: FastAPI request
+
+    Returns:
+        ContentType enum value
+    """
+    if wants_activity_json(request):
+        return ContentType.ACTIVITY_JSON
+    if wants_json(request):
+        return ContentType.JSON
+    return ContentType.HTML
+
+
+def is_htmx_request(request: Request) -> bool:
+    """
+    Check if this is an HTMX request (partial page update).
+
+    HTMX requests set the HX-Request header.
+
+    Args:
+        request: FastAPI request
+
+    Returns:
+        True if this is an HTMX request
+    """
+    return request.headers.get("HX-Request") == "true"
+
+
+def get_htmx_target(request: Request) -> Optional[str]:
+    """
+    Get the HTMX target element ID.
+
+    Args:
+        request: FastAPI request
+
+    Returns:
+        Target element ID or None
+    """
+    return request.headers.get("HX-Target")
+
+
+def get_htmx_trigger(request: Request) -> Optional[str]:
+    """
+    Get the HTMX trigger element ID.
+
+    Args:
+        request: FastAPI request
+
+    Returns:
+        Trigger element ID or None
+    """
+    return request.headers.get("HX-Trigger")
+
+
+def is_ios_request(request: Request) -> bool:
+    """
+    Check if request is from iOS device.
+
+    Useful for video format selection (iOS prefers MP4).
+
+    Args:
+        request: FastAPI request
+
+    Returns:
+        True if iOS user agent detected
+    """
+    user_agent = request.headers.get("user-agent", "").lower()
+    return "iphone" in user_agent or "ipad" in user_agent