feat(blog): Phase 5 SEO/GEO content with 5 new blog posts

Add 5 long-form Thai blog posts (1,200-2,500 words each) with SEO + GEO
optimization for the dealplustech water-systems site. Each post targets
a specific audience (contractors, engineers, project managers) and
follows a content-quality workflow: source real product specs, verify
Thai text, dedupe images, link back to product pages.

## New blog posts (src/content/blog/)
- thermobreak-guide.md (Thermobreak closed-cell insulation overview)
- plastic-grilles-guide.md (ABS plastic grilles for HVAC)
- ppr-pipe-guide.md (PPR pipe properties + heat-fusion welding)
- ppr-vs-hdpe-vs-upvc.md (3-way pipe comparison with PE80/PE100)
- thermobreak-series-guide.md (Thermobreak LS vs Solar series)
- 10-things-checklist-pipe-ordering.md (10-point pre-order checklist)

## Removed legacy posts
- pipe-knowledge.md, valve-guide.md, welcome-post.md (orphans)

## Hero images (public/images/blog/)
~20 product photos sourced from manufacturers (Thermobreak, Thai PPR,
thaiconsupply) plus Nano Banana Pro infographics. All resized to
3:2 aspect ratio per user preference. Source folder preserved for
re-derivation.

## Astro layout/SEO work
- src/components/seo/SEO.astro, JsonLd.astro (new SEO components)
- src/layouts/BaseLayout.astro, Layout.astro (OG/Twitter/JSON-LD wiring)
- src/pages/404.astro
- Product pages (8): added #pricelist anchors + schema work
- src/styles/global.css: scroll-padding for sticky-header anchors

## Automation scripts (scripts/)
- build_og_image.py (OG image builder)
- inject_faq_schema.py, inject_product_schema.py (JSON-LD injection)

## Misc
- public/robots.txt, public/images/og/default-og.jpg
- .gitignore: exclude scripts/__pycache__/

src/pages/sitemap.xml.ts

@@ -0,0 +1,153 @@
+/**
+ * /sitemap.xml.ts — Custom sitemap with per-type priority/changefreq.
+ *
+ * Generates a sitemap by:
+ *   1. Listing all static .astro pages (excluding 404 + dynamic [slug])
+ *   2. Walking content collections (blog) for dynamic routes
+ *   3. Classifying each URL into a category to assign priority + changefreq
+ *
+ * Per Astro 6, the file lives in src/pages/ and exports a GET handler.
+ */
+import type { APIRoute } from 'astro';
+import { getCollection } from 'astro:content';
+
+const SITE = 'https://dealplustech.com';
+
+// ---------------------------------------------------------------------------
+// URL classification
+// ---------------------------------------------------------------------------
+
+const NOINDEX_PAGES = new Set([
+  'privacy-policy',
+  'terms-and-conditions',
+  '404',
+]);
+
+const PRODUCT_SLUGS = new Set([
+  'aeroflex', 'armflex', 'durgo-avvs', 'grilles', 'maxflex',
+  'pipe-coupling', 'realflex', 'water-pump', 'water-treatment',
+  'ตู้ดับเพลิง',
+  'ท่อ-hdpe', 'ท่อ-ppr-scg', 'ท่อ-ppr-thai-ppr', 'ท่อ-syler',
+  'ท่อ-upvc', 'ท่อ-xy-lent',
+  'ระบบรั้วไวน์แมน', 'รั้วเทวดา',
+  'วาล์ว-valve', 'หัวจ่าย-ball-jet',
+  'เครื่องเชื่อม-hdpe', 'เครื่องเชื่อม-ppr',
+  'เทอร์โมเบรค-thermobreak', 'เม็กกรู๊ฟ-คับปลิ้ง',
+]);
+
+const HOMEPAGE_SLUGS = new Set(['index', 'all-products']);
+const CATEGORY_SLUGS = new Set(['ระบบน้ำ']);
+const STATIC_SLUGS = new Set(['about-us', 'contact-us', 'portfolio']);
+
+type Entry = {
+  loc: string;
+  priority: number;
+  changefreq: 'daily' | 'weekly' | 'monthly' | 'yearly';
+  lastmod?: string;
+};
+
+function classify(slug: string): { priority: number; changefreq: Entry['changefreq'] } {
+  if (slug === 'index') return { priority: 1.0, changefreq: 'weekly' };
+  if (slug === 'all-products') return { priority: 0.9, changefreq: 'weekly' };
+  if (PRODUCT_SLUGS.has(slug)) return { priority: 0.9, changefreq: 'monthly' };
+  if (CATEGORY_SLUGS.has(slug)) return { priority: 0.8, changefreq: 'monthly' };
+  if (STATIC_SLUGS.has(slug)) return { priority: 0.6, changefreq: 'yearly' };
+  return { priority: 0.5, changefreq: 'monthly' };
+}
+
+function urlFromSlug(slug: string): string {
+  if (slug === 'index') return `${SITE}/`;
+  return `${SITE}/${slug}`;
+}
+
+// ---------------------------------------------------------------------------
+// Build entries
+// ---------------------------------------------------------------------------
+
+async function buildEntries(): Promise<Entry[]> {
+  // 1. Static pages — discovered via Vite's import.meta.glob
+  const modules = import.meta.glob<true>('./*.astro');
+  const pageFiles = Object.keys(modules)
+    .map(p => p.replace(/^\.\//, '').replace(/\.astro$/, ''))
+    .filter(slug => !slug.startsWith('[') && !NOINDEX_PAGES.has(slug));
+
+  const entries: Entry[] = pageFiles.map(slug => {
+    const { priority, changefreq } = classify(slug);
+    return { loc: urlFromSlug(slug), priority, changefreq };
+  });
+
+  // 2. Blog posts (dynamic [slug] route)
+  const articles = await getCollection('blog');
+  for (const article of articles) {
+    entries.push({
+      loc: `${SITE}/${encodeURI('บทความ')}/${encodeURIComponent(article.id)}`,
+      priority: 0.7,
+      changefreq: 'yearly',
+      lastmod: article.data.published_at.toISOString(),
+    });
+  }
+
+  // 3. Blog index
+  entries.push({
+    loc: `${SITE}/${encodeURI('บทความ')}`,
+    priority: 0.7,
+    changefreq: 'weekly',
+  });
+
+  // Sort: homepage first, then by priority desc, then by URL asc
+  entries.sort((a, b) => {
+    if (a.loc === `${SITE}/`) return -1;
+    if (b.loc === `${SITE}/`) return 1;
+    if (a.priority !== b.priority) return b.priority - a.priority;
+    return a.loc.localeCompare(b.loc);
+  });
+
+  return entries;
+}
+
+// ---------------------------------------------------------------------------
+// XML serialiser
+// ---------------------------------------------------------------------------
+
+function escapeXml(s: string): string {
+  return s
+    .replace(/&/g, '&amp;')
+    .replace(/</g, '&lt;')
+    .replace(/>/g, '&gt;')
+    .replace(/"/g, '&quot;')
+    .replace(/'/g, '&apos;');
+}
+
+function toXml(entries: Entry[]): string {
+  const urls = entries.map(e => {
+    const lastmod = e.lastmod ? `\n    <lastmod>${e.lastmod}</lastmod>` : '';
+    return `  <url>
+    <loc>${escapeXml(e.loc)}</loc>${lastmod}
+    <changefreq>${e.changefreq}</changefreq>
+    <priority>${e.priority.toFixed(1)}</priority>
+  </url>`;
+  }).join('\n');
+
+  return `<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+${urls}
+</urlset>
+`;
+}
+
+// ---------------------------------------------------------------------------
+// Handler
+// ---------------------------------------------------------------------------
+
+export const GET: APIRoute = async () => {
+  const entries = await buildEntries();
+  const xml = toXml(entries);
+
+  return new Response(xml, {
+    status: 200,
+    headers: {
+      'Content-Type': 'application/xml; charset=utf-8',
+      'Cache-Control': 'public, max-age=3600',
+    },
+  });
+};