Add fragment blueprint + sync shared: micro-frontend infrastructure

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

bp/__init__.py

@@ -1,2 +1,3 @@
 from .identity.routes import register as register_identity_bp
 from .social.routes import register as register_social_bp
+from .fragments import register_fragments

bp/fragments/__init__.py

@@ -0,0 +1 @@
+from .routes import register as register_fragments

bp/fragments/routes.py

@@ -0,0 +1,34 @@
+"""Federation app fragment endpoints.
+
+Exposes HTML fragments at ``/internal/fragments/<type>`` for consumption
+by other coop apps via the fragment client.
+"""
+
+from __future__ import annotations
+
+from quart import Blueprint, Response, request
+
+from shared.infrastructure.fragments import FRAGMENT_HEADER
+
+
+def register():
+    bp = Blueprint("fragments", __name__, url_prefix="/internal/fragments")
+
+    _handlers: dict[str, object] = {}
+
+    @bp.before_request
+    async def _require_fragment_header():
+        if not request.headers.get(FRAGMENT_HEADER):
+            return Response("", status=403)
+
+    @bp.get("/<fragment_type>")
+    async def get_fragment(fragment_type: str):
+        handler = _handlers.get(fragment_type)
+        if handler is None:
+            return Response("", status=200, content_type="text/html")
+        html = await handler()
+        return Response(html, status=200, content_type="text/html")
+
+    bp._fragment_handlers = _handlers
+
+    return bp