Add fragment router for micro-frontend composition

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/routers/fragments.py

@@ -0,0 +1,27 @@
+"""
+Art-DAG fragment endpoints.
+
+Exposes HTML fragments at ``/internal/fragments/{type}`` for consumption
+by coop apps via the fragment client.
+"""
+
+from fastapi import APIRouter, Request, Response
+
+router = APIRouter()
+
+# Registry of fragment handlers: type -> async callable(request) returning HTML str
+_handlers: dict[str, object] = {}
+
+FRAGMENT_HEADER = "X-Fragment-Request"
+
+
+@router.get("/internal/fragments/{fragment_type}")
+async def get_fragment(fragment_type: str, request: Request):
+    if not request.headers.get(FRAGMENT_HEADER):
+        return Response(content="", status_code=403)
+
+    handler = _handlers.get(fragment_type)
+    if handler is None:
+        return Response(content="", media_type="text/html", status_code=200)
+    html = await handler(request)
+    return Response(content=html, media_type="text/html", status_code=200)