feat: Pro limit 2,500/mo, website templating, cleanup

- Set Pro tier limit to 2,500 PDFs/month (was unlimited/5000)
- Added Pro limit enforcement in usage middleware
- Updated landing page, JSON-LD, and Stripe product description
- Created build-time HTML templating (partials for nav/footer/styles)
- Source files in public/src/, partials in public/partials/
- Build script: node scripts/build-html.cjs
- Deleted stale backup file
- Fixed index.html nav logo to use <a> tag for consistency

public/partials/_footer.html

@@ -0,0 +1,14 @@
+<footer aria-label="Footer">
+  <div class="container">
+    <div class="footer-left">© 2026 DocFast. Fast PDF generation for developers.</div>
+    <div class="footer-links">
+      <a href="/">Home</a>
+      <a href="/docs">Docs</a>
+      <a href="/health">API Status</a>
+      <a href="/#change-email" class="open-email-change">Change Email</a>
+      <a href="/impressum">Impressum</a>
+      <a href="/privacy">Privacy Policy</a>
+      <a href="/terms">Terms of Service</a>
+    </div>
+  </div>
+</footer>