Add /quantity/ endpoint so cart +/- buttons work same-origin

Adds POST /quantity/<product_id>/ to set cart item quantity (or remove at 0),
and registers cart_quantity_url Jinja global so the shared template uses it.

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

app.py

@@ -99,6 +99,8 @@ def create_app() -> "Quart":
         app.jinja_loader,
     ])
 
+    app.jinja_env.globals["cart_quantity_url"] = lambda product_id: f"/quantity/{product_id}/"
+
     # --- Page slug hydration (follows events/market app pattern) ---
 
     @app.url_value_preprocessor

bp/cart/global_routes.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 from quart import Blueprint, g, request, render_template, redirect, url_for, make_response
 from sqlalchemy import select
 
+from shared.models.market import CartItem
 from shared.models.order import Order
 from shared.models.market_place import MarketPlace
 from shared.services.registry import services
@@ -53,6 +54,34 @@ def register(url_prefix: str) -> Blueprint:
 
         return redirect(url_for("cart_overview.overview"))
 
+    @bp.post("/quantity/<int:product_id>/")
+    async def update_quantity(product_id: int):
+        ident = current_cart_identity()
+        form = await request.form
+        count = int(form.get("count", 0))
+
+        filters = [
+            CartItem.deleted_at.is_(None),
+            CartItem.product_id == product_id,
+        ]
+        if ident["user_id"] is not None:
+            filters.append(CartItem.user_id == ident["user_id"])
+        else:
+            filters.append(CartItem.session_id == ident["session_id"])
+
+        existing = await g.s.scalar(select(CartItem).where(*filters))
+
+        if existing:
+            if count <= 0:
+                await g.s.delete(existing)
+            else:
+                existing.quantity = count
+            await g.s.flush()
+
+        resp = await make_response("", 200)
+        resp.headers["HX-Refresh"] = "true"
+        return resp
+
     @bp.post("/checkout/")
     async def checkout():
         """Legacy global checkout (for orphan items without page scope)."""