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/_nav.html

@@ -0,0 +1,10 @@
+<nav aria-label="Main navigation">
+  <div class="container">
+    <a href="/" class="logo">⚡ Doc<span>Fast</span></a>
+    <div class="nav-links">
+      <a href="/#features">Features</a>
+      <a href="/#pricing">Pricing</a>
+      <a href="/docs">Docs</a>
+    </div>
+  </div>
+</nav>