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

dist/routes/billing.js

@@ -67,7 +67,7 @@ a { color: #4f9; }
 <p>Your API key:</p>
 <div class="key" style="position:relative">${escapeHtml(keyInfo.key)}<button onclick="navigator.clipboard.writeText('${escapeHtml(keyInfo.key)}');this.textContent='Copied!';setTimeout(()=>this.textContent='Copy',1500)" style="position:absolute;top:8px;right:8px;background:#4f9;color:#0a0a0a;border:none;border-radius:4px;padding:4px 12px;cursor:pointer;font-size:0.8rem;font-family:system-ui">Copy</button></div>
 <p><strong>Save this key!</strong> It won't be shown again.</p>
-<p>10,000 PDFs/month • All endpoints • Priority support</p>
+<p>5,000 PDFs/month • All endpoints • Priority support</p>
 <p><a href="/docs">View API docs →</a></p>
 </div></body></html>`);
     }
@@ -171,7 +171,7 @@ async function getOrCreateProPrice() {
     else {
         const product = await getStripe().products.create({
             name: "DocFast Pro",
-            description: "Unlimited PDF conversions via API. HTML, Markdown, and URL to PDF.",
+            description: "5,000 PDFs / month via API. HTML, Markdown, and URL to PDF.",
         });
         productId = product.id;
     }