feat: add developer blog with two posts

- Blog index page (public/blog.html) with dark theme
- Post 1: Why You Need a Screenshot API (~800 words)
- Post 2: Screenshot API Performance & Caching (~600 words)
- Express routes: /blog → /blog.html, /blog/:slug → /blog/:slug.html
- Blog link added to nav and footer on index.html
- Sitemap updated with blog URLs
- Full test coverage (19 new tests, 190 total passing)

public/index.html

@@ -246,6 +246,7 @@ footer{border-top:1px solid var(--border);padding:48px 24px 32px;background:var(
       <a href="/usage">Usage</a>
       <a href="/compare">Compare</a>
       <a href="/guides/quick-start">Quick Start</a>
+      <a href="/blog">Blog</a>
       <a href="#pricing" class="btn btn-primary btn-sm nav-cta">Get API Key</a>
     </div>
     <button class="nav-mobile" onclick="document.querySelector('.nav-links').classList.toggle('show')" aria-label="Menu">☰</button>
@@ -739,6 +740,7 @@ screenshot = snap.<span class="fn">capture</span>(
       <a href="/health">Status</a>
       <a href="/usage">Usage Dashboard</a>
       <a href="/changelog">Changelog</a>
+      <a href="/blog">Blog</a>
     </div>
     <div class="footer-col">
       <h5>Legal</h5>