fix: remove duplicate market_slug in blueprint registration

The url_prefix was being set both in register_market_bp() and
app.register_blueprint(), causing a SyntaxError from duplicate
route parameter.

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

app.py

@@ -57,7 +57,7 @@ def create_app() -> "Quart":
     # Market blueprint scoped under /<market_slug>/
     app.register_blueprint(
         register_market_bp(
-            url_prefix="/<market_slug>",
+            url_prefix="/",
             title=config()["coop_title"],
         ),
         url_prefix="/<market_slug>",