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/_styles_base.html

@@ -0,0 +1,37 @@
+<style>
+*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
+:root {
+  --bg: #0b0d11; --bg2: #12151c; --fg: #e4e7ed; --muted: #7a8194;
+  --accent: #34d399; --accent-hover: #5eead4; --accent-glow: rgba(52,211,153,0.12);
+  --card: #151922; --border: #1e2433;
+  --radius: 12px; --radius-lg: 16px;
+}
+body { font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif; background: var(--bg); color: var(--fg); line-height: 1.65; -webkit-font-smoothing: antialiased; }
+a { color: var(--accent); text-decoration: none; transition: color 0.2s; }
+a:hover { color: var(--accent-hover); }
+.container { max-width: 800px; margin: 0 auto; padding: 0 24px; }
+nav { padding: 20px 0; border-bottom: 1px solid var(--border); }
+nav .container { display: flex; align-items: center; justify-content: space-between; }
+.logo { font-size: 1.25rem; font-weight: 700; letter-spacing: -0.5px; color: var(--fg); display: flex; align-items: center; gap: 8px; text-decoration: none; }
+.logo span { color: var(--accent); }
+.nav-links { display: flex; gap: 28px; align-items: center; }
+.nav-links a { color: var(--muted); font-size: 0.9rem; font-weight: 500; }
+.nav-links a:hover { color: var(--fg); }
+.content { padding: 60px 0; min-height: 60vh; }
+.content h1 { font-size: 2rem; font-weight: 800; margin-bottom: 32px; letter-spacing: -1px; }
+.content h2 { font-size: 1.3rem; font-weight: 700; margin: 32px 0 16px; color: var(--fg); }
+.content h3 { font-size: 1.1rem; font-weight: 600; margin: 24px 0 12px; color: var(--fg); }
+.content p, .content li { color: var(--muted); margin-bottom: 12px; }
+.content ul, .content ol { padding-left: 24px; }
+.content strong { color: var(--fg); }
+footer { padding: 32px 0; border-top: 1px solid var(--border); margin-top: 60px; }
+footer .container { display: flex; justify-content: space-between; align-items: center; flex-wrap: wrap; gap: 16px; }
+.footer-left { color: var(--muted); font-size: 0.85rem; }
+.footer-links { display: flex; gap: 20px; flex-wrap: wrap; }
+.footer-links a { color: var(--muted); font-size: 0.85rem; }
+.footer-links a:hover { color: var(--fg); }
+@media (max-width: 768px) {
+  footer .container { flex-direction: column; text-align: center; }
+  .nav-links { gap: 16px; }
+}
+</style>