[Phase 5] docs-site: add VIN Decoder Redoc page and SEO blog post

- Add docs-site/src/pages/vin-decoder.astro: Redoc reference page
  rendering /specs/vin-decoder.yaml (matches pattern of existing API pages)
- Copy apis/vin-decoder/openapi.yaml to docs-site/public/specs/vin-decoder.yaml
  so Redoc can serve the spec
- Add VIN Decoder card to index.astro hero grid (4th API)
- Add docs-site/src/pages/blog/vin-decoder.mdx: 1,500+ word SEO blog post
  'How to Decode a VIN Number with Node.js Using Free NHTSA Data'
  with meta description, working code examples, VIN structure explanation,
  NHTSA vPIC background, and use-case context
- npm run build exits 0; /vin-decoder and /blog/vin-decoder routes present

Closes leeworks-agents/api-company#123
Closes leeworks-agents/api-company#124

docs-site/src/pages/vin-decoder.astro

@@ -0,0 +1,27 @@
+---
+import Base from '../layouts/Base.astro';
+
+const apiName = 'vin-decoder';
+const title = 'VIN Decoder API';
+const description = 'Decode any 17-character VIN into make, model, year, trim, engine, body class, and more. Powered by the NHTSA vPIC public-domain database.';
+---
+<Base title={title} description={description}>
+  <style>
+    #redoc-container { background: #fff; }
+  </style>
+  <div id="redoc-container"></div>
+  <script is:inline define:vars={{ specUrl: `/specs/${apiName}.yaml` }}>
+    // Load Redoc from CDN
+    var script = document.createElement('script');
+    script.src = 'https://cdn.jsdelivr.net/npm/redoc@latest/bundles/redoc.standalone.js';
+    script.onload = function () {
+      Redoc.init(specUrl, {
+        theme: {
+          colors: { primary: { main: '#667eea' } },
+          typography: { fontFamily: 'system-ui, sans-serif' },
+        },
+      }, document.getElementById('redoc-container'));
+    };
+    document.head.appendChild(script);
+  </script>
+</Base>