Add global + page-scoped market listings with infinite scroll

- New all_markets blueprint at / with paginated grid and HTMX infinite scroll
- New page_markets blueprint at /<slug>/ for page-scoped market listing
- list_marketplaces service method (via shared submodule update)
- Updated slug preprocessor to handle both /<slug>/ and /<page_slug>/<market_slug>/
- Removed inline markets_listing() route (replaced by all_markets blueprint)

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

bp/page_markets/routes.py

@@ -0,0 +1,65 @@
+"""
+Page-markets blueprint — shows markets for a single page.
+
+Mounted at /<slug> (page-scoped). Requires g.post_data from hydrate_post.
+
+Routes:
+  GET  /<slug>/              — full page scoped to this page
+  GET  /<slug>/page-markets  — HTMX fragment for infinite scroll
+"""
+from __future__ import annotations
+
+from quart import Blueprint, g, request, render_template, make_response
+
+from shared.browser.app.utils.htmx import is_htmx_request
+from shared.services.registry import services
+
+
+def register() -> Blueprint:
+    bp = Blueprint("page_markets", __name__)
+
+    async def _load_markets(post_id, page, per_page=20):
+        """Load markets for this page's container."""
+        markets, has_more = await services.market.list_marketplaces(
+            g.s, "page", post_id, page=page, per_page=per_page,
+        )
+        return markets, has_more
+
+    @bp.get("/")
+    async def index():
+        post = g.post_data["post"]
+        page = int(request.args.get("page", 1))
+
+        markets, has_more = await _load_markets(post["id"], page)
+
+        ctx = dict(
+            markets=markets,
+            has_more=has_more,
+            page_info={},
+            page=page,
+        )
+
+        if is_htmx_request():
+            html = await render_template("_types/page_markets/_main_panel.html", **ctx)
+        else:
+            html = await render_template("_types/page_markets/index.html", **ctx)
+
+        return await make_response(html, 200)
+
+    @bp.get("/page-markets")
+    async def markets_fragment():
+        post = g.post_data["post"]
+        page = int(request.args.get("page", 1))
+
+        markets, has_more = await _load_markets(post["id"], page)
+
+        html = await render_template(
+            "_types/page_markets/_cards.html",
+            markets=markets,
+            has_more=has_more,
+            page_info={},
+            page=page,
+        )
+        return await make_response(html, 200)
+
+    return bp