feat(web): add SEO optimization — JSON-LD, dynamic sitemap, meta tags for listings

Add comprehensive SEO support for property listing pages to improve
organic search visibility and social sharing.

Changes:
- Convert listing detail page from client-only to server component wrapper
  with generateMetadata() for per-listing title, description, OG tags,
  canonical URLs, and hreflang alternates
- Add JSON-LD structured data (Schema.org RealEstateListing) with price,
  location, property specs, and breadcrumb markup
- Add Website JSON-LD with SearchAction to root layout
- Upgrade sitemap.xml to dynamically include all active listings across
  both locales (vi, en) with ISR revalidation
- Improve robots.txt with pagination/sort exclusions and GPTBot block
- Create server-side fetch utility (listings-server.ts) for SSR data
- Extract client UI into ListingDetailClient component

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/lib/listings-server.ts

@@ -0,0 +1,51 @@
+/**
+ * Server-side listing data fetching for Next.js Server Components.
+ *
+ * This module uses `fetch` directly (no browser-only helpers) so it can run
+ * inside `generateMetadata`, `generateStaticParams`, `sitemap()`, etc.
+ */
+
+import type { ListingDetail, PaginatedResult } from './listings-api';
+
+const API_BASE_URL = process.env['NEXT_PUBLIC_API_URL'] || 'http://localhost:3001/api/v1';
+
+/**
+ * Fetch a single listing by ID — server-only.
+ * Returns `null` when the listing is not found (404) so callers can `notFound()`.
+ */
+export async function fetchListingById(id: string): Promise<ListingDetail | null> {
+  try {
+    const res = await fetch(`${API_BASE_URL}/listings/${id}`, {
+      next: { revalidate: 300 }, // ISR: re-validate every 5 min
+    });
+
+    if (!res.ok) return null;
+    return (await res.json()) as ListingDetail;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Fetch active listings — server-only, used by the dynamic sitemap.
+ */
+export async function fetchActiveListings(params: {
+  page?: number;
+  limit?: number;
+}): Promise<PaginatedResult<ListingDetail>> {
+  const query = new URLSearchParams({
+    status: 'ACTIVE',
+    page: String(params.page ?? 1),
+    limit: String(params.limit ?? 100),
+  });
+
+  const res = await fetch(`${API_BASE_URL}/listings?${query}`, {
+    next: { revalidate: 3600 }, // re-validate every hour for sitemap
+  });
+
+  if (!res.ok) {
+    return { data: [], total: 0, page: 1, limit: 100, totalPages: 0 };
+  }
+
+  return (await res.json()) as PaginatedResult<ListingDetail>;
+}