v0.4.0: Remove free tier, add public demo endpoint with watermark

- Remove free account signup flow entirely
- Add POST /v1/demo/html and /v1/demo/markdown (public, no auth)
- Demo: 5 requests/hour per IP, 50KB body limit, watermarked PDFs
- Landing page: interactive playground replaces 'Get Free API Key'
- Pricing: Demo (free) + Pro (€9/mo), no more Free tier
- /v1/signup returns 410 Gone with redirect to demo/pro
- Keep /v1/recover for existing Pro users
- Update JSON-LD, API discovery, verify page text

package.json

@@ -1,6 +1,6 @@
 {
   "name": "docfast-api",
-  "version": "0.3.4",
+  "version": "0.4.0",
   "description": "Markdown/HTML to PDF API with built-in invoice templates",
   "main": "dist/index.js",
   "scripts": {