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__/

public/robots.txt

@@ -0,0 +1,41 @@
+# robots.txt — ดีล พลัส เทค
+# https://dealplustech.com
+
+User-agent: *
+Allow: /
+Disallow: /404
+
+# AI crawlers — เปิดให้อ่าน (ช่วย GEO)
+User-agent: GPTBot
+Allow: /
+
+User-agent: Google-Extended
+Allow: /
+
+User-agent: PerplexityBot
+Allow: /
+
+User-agent: anthropic-ai
+Allow: /
+
+User-agent: ClaudeBot
+Allow: /
+
+User-agent: Claude-Web
+Allow: /
+
+# Bad bots — block common scrapers that don't respect crawl-delay
+User-agent: AhrefsBot
+Disallow: /
+
+User-agent: SemrushBot
+Disallow: /
+
+User-agent: MJ12bot
+Disallow: /
+
+# Host (Yandex/Bing-style, also helps some other crawlers)
+Host: https://dealplustech.com
+
+# Sitemap
+Sitemap: https://dealplustech.com/sitemap.xml