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__/

scripts/inject_faq_schema.py

@@ -0,0 +1,152 @@
+"""
+Extract FAQ Q&A pairs from product pages and inject as `faq={[...]}` prop.
+
+Pattern detected:
+  <h3 ...>Q: ...question...</h3>
+  <p ...>...answer...</p>
+
+Only operates on pages that have a FAQ section (search for 'คำถามที่พบบ่อย').
+"""
+import re
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parent.parent
+PAGES = ROOT / "src" / "pages"
+
+# Product pages with FAQ UI (from earlier audit: lines >= 240 and grep -c FAQ > 0)
+TARGETS = [
+    "pipe-coupling.astro",
+    "ท่อ-syler.astro",
+    "หัวจ่าย-ball-jet.astro",
+    "เม็กกรู๊ฟ-คับปลิ้ง.astro",
+    "เครื่องเชื่อม-hdpe.astro",
+    "เครื่องเชื่อม-ppr.astro",
+    "เทอร์โมเบรค-thermobreak.astro",
+    "realflex.astro",
+    "water-treatment.astro",
+    "วาล์ว-valve.astro",
+    "รั้วเทวดา.astro",
+    "ระบบรั้วไวน์แมน.astro",
+    "durgo-avvs.astro",
+    "ตู้ดับเพลิง.astro",
+    "water-pump.astro",
+    "grilles.astro",
+    "ท่อ-upvc.astro",
+    "armflex.astro",
+    "aeroflex.astro",
+    "maxflex.astro",
+]
+
+
+def extract_faq(content: str) -> list[tuple[str, str]]:
+    """Return list of (question, answer) tuples from FAQ section.
+
+    Scopes regex to the FAQ section only: starts at "คำถามที่พบบ่อย" and
+    ends at the next "<!-- Contact CTA" or "Contact CTA" marker. This
+    prevents pattern D from catching feature lists elsewhere on the page.
+
+    Handles 4 patterns:
+      A. <h3>Q: ...</h3><p>...</p>
+      B. <h3>1. ...</h3><p>...</p>   (number prefix)
+      C. <details><summary>...</summary><div>...</div></details>
+      D. <h3>question</h3><p>answer</p>   (no prefix — grilles style)
+    """
+    if 'คำถามที่พบบ่อย' not in content:
+        return []
+
+    # Slice content to the FAQ block: from "คำถามที่พบบ่อย" up to the next
+    # Contact CTA / section end. Fall back to end-of-file if no terminator.
+    start = content.index('คำถามที่พบบ่อย')
+    end = len(content)
+    for marker in ('<!-- Contact CTA', 'Contact CTA', '<!-- End FAQ', '</section>\n    <!--'):
+        idx = content.find(marker, start)
+        if idx != -1 and idx < end:
+            end = idx
+    block = content[start:end]
+
+    pairs: list[tuple[str, str]] = []
+
+    # Pattern A + B: <h2-h4 ...>prefix...</h2-h4><p>answer</p>
+    p_ab = re.compile(
+        r'<h[234]\b[^>]*>\s*(?:Q:|\d+\.\s*)(?P<q>[^<]+?)\s*</h[234]>\s*<p\b[^>]*>(?P<a>.*?)</p>',
+        re.DOTALL,
+    )
+    for m in p_ab.finditer(block):
+        pairs.append((m.group('q').strip(), m.group('a').strip()))
+
+    if not pairs:
+        # Pattern D: <h2-h4>question</h2-h4><p>answer</p> (no prefix)
+        p_d = re.compile(
+            r'<h[234]\b[^>]*>\s*(?P<q>[^<]+?)\s*</h[234]>\s*<p\b[^>]*>(?P<a>.*?)</p>',
+            re.DOTALL,
+        )
+        for m in p_d.finditer(block):
+            pairs.append((m.group('q').strip(), m.group('a').strip()))
+
+    if not pairs:
+        # Pattern C: <details><summary>...</summary><div>...</div></details>
+        p_c = re.compile(
+            r'<details\b[^>]*>\s*<summary\b[^>]*>(?P<sum>.*?)</summary>'
+            r'.*?<div\b[^>]*>(?P<a>.*?)</div>\s*</details>',
+            re.DOTALL,
+        )
+        for m in p_c.finditer(block):
+            sum_html = m.group('sum')
+            sp = re.search(r'<span\b[^>]*>(?P<q>.*?)</span>', sum_html, re.DOTALL)
+            q = sp.group('q').strip() if sp else re.sub(r'<[^>]+>', '', sum_html).strip()
+            pairs.append((q, m.group('a').strip()))
+
+    return pairs
+
+
+def js_string(value: str) -> str:
+    """Single-quoted JS literal that's safe to embed in Astro JSX."""
+    return "'" + value.replace("\\", "\\\\").replace("'", "\\'") + "'"
+
+
+def build_faq_prop(pairs: list[tuple[str, str]]) -> str:
+    lines = ['\n  faq={[']
+    for q, a in pairs:
+        lines.append(f'    {{ question: {js_string(q)}, answer: {js_string(a)} }},')
+    lines.append('  ]}')
+    return '\n'.join(lines)
+
+
+def inject_prop(content: str, prop_block: str) -> str:
+    """Insert prop_block before the closing > of the first <BaseLayout> tag."""
+    m = re.search(r'<BaseLayout\b[^>]*>', content)
+    if not m:
+        return content
+    insert_at = m.end() - 1
+    return content[:insert_at] + prop_block + content[insert_at:]
+
+
+def process_file(path: Path) -> str:
+    content = path.read_text(encoding='utf-8')
+    # Idempotent: if faq prop already exists, skip.
+    if 'faq={[' in content:
+        return f"SKIP (already has faq prop): {path.name}"
+    pairs = extract_faq(content)
+    if not pairs:
+        return f"SKIP (no FAQ): {path.name}"
+
+    prop_block = build_faq_prop(pairs)
+    new_content = inject_prop(content, prop_block)
+    if new_content == content:
+        return f"NO-CHANGE: {path.name}"
+
+    path.write_text(new_content, encoding='utf-8')
+    return f"OK ({len(pairs)} pairs): {path.name}"
+
+
+def main() -> None:
+    for name in TARGETS:
+        path = PAGES / name
+        if not path.exists():
+            print(f"MISSING: {name}")
+            continue
+        print(process_file(path))
+
+
+if __name__ == "__main__":
+    main()