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

package.json

@@ -4,6 +4,7 @@
   "description": "Markdown/HTML to PDF API with built-in invoice templates",
   "main": "dist/index.js",
   "scripts": {
+    "build:html": "node scripts/build-html.cjs",
     "build": "tsc",
     "start": "node dist/index.js",
     "dev": "tsx src/index.ts",