Phase 4: add container-nav fragment handler for market links

Market app serves marketplace links as a fragment at
/internal/fragments/container-nav for consumption by blog and events.

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

bp/fragments/routes.py

@@ -6,9 +6,10 @@ by other coop apps via the fragment client.
 
 from __future__ import annotations
 
-from quart import Blueprint, Response, request
+from quart import Blueprint, Response, g, render_template, request
 
 from shared.infrastructure.fragments import FRAGMENT_HEADER
+from shared.services.registry import services
 
 
 def register():
@@ -29,6 +30,25 @@ def register():
         html = await handler()
         return Response(html, status=200, content_type="text/html")
 
+    # --- container-nav fragment: market links --------------------------------
+
+    async def _container_nav_handler():
+        container_type = request.args.get("container_type", "page")
+        container_id = int(request.args.get("container_id", 0))
+        post_slug = request.args.get("post_slug", "")
+
+        markets = await services.market.marketplaces_for_container(
+            g.s, container_type, container_id,
+        )
+        if not markets:
+            return ""
+        return await render_template(
+            "fragments/container_nav_markets.html",
+            markets=markets, post_slug=post_slug,
+        )
+
+    _handlers["container-nav"] = _container_nav_handler
+
     bp._fragment_handlers = _handlers
 
     return bp