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/index.html

@@ -17,7 +17,7 @@
 <link rel="preconnect" href="https://fonts.googleapis.com">
 <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
 <script type="application/ld+json">
-{"@context":"https://schema.org","@type":"SoftwareApplication","name":"DocFast","url":"https://docfast.dev","applicationCategory":"DeveloperApplication","operatingSystem":"Web","description":"Convert HTML and Markdown to beautiful PDFs with a simple API call. Fast, reliable, developer-friendly.","offers":[{"@type":"Offer","price":"0","priceCurrency":"EUR","name":"Free","description":"100 PDFs/month"},{"@type":"Offer","price":"9","priceCurrency":"EUR","name":"Pro","description":"5,000 PDFs / month","billingIncrement":"P1M"}]}
+{"@context":"https://schema.org","@type":"SoftwareApplication","name":"DocFast","url":"https://docfast.dev","applicationCategory":"DeveloperApplication","operatingSystem":"Web","description":"Convert HTML and Markdown to beautiful PDFs with a simple API call. Fast, reliable, developer-friendly.","offers":[{"@type":"Offer","price":"0","priceCurrency":"EUR","name":"Free","description":"100 PDFs/month"},{"@type":"Offer","price":"9","priceCurrency":"EUR","name":"Pro","description":"2,500 PDFs per month","billingIncrement":"P1M"}]}
 </script>
 <link rel="icon" href="data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'><text y='.9em' font-size='90'>⚡</text></svg>">
 <style>
@@ -271,7 +271,7 @@ html, body {
 
 <nav aria-label="Main navigation">
   <div class="container">
-    <div class="logo">⚡ Doc<span>Fast</span></div>
+    <a href="/" class="logo">⚡ Doc<span>Fast</span></a>
     <div class="nav-links">
       <a href="#features">Features</a>
       <a href="#pricing">Pricing</a>
@@ -404,7 +404,7 @@ html, body {
         <div class="price-amount">€9<span> /mo</span></div>
         <div class="price-desc">For production apps and businesses</div>
         <ul class="price-features">
-          <li>5,000 PDFs / month</li>
+<li>2,500 PDFs per month</li>
           <li>All conversion endpoints</li>
           <li>All templates included</li>
           <li>Priority support</li>