docs: remove free tier, update rate limits and auth for demo+pro model

- Remove free tier from rate limits, add Demo (5/hour, watermarked)
- Update auth section: remove free-tier key mention, link to docfast.dev
- Update getting started: demo → upgrade to Pro → use API key
- Add deprecated: true to /v1/signup/free swagger annotation
- Regenerate openapi.json

src/index.ts

@@ -104,6 +104,7 @@ app.use("/v1/demo", express.json({ limit: "50kb" }), pdfRateLimitMiddleware, dem
  * /v1/signup/free:
  *   post:
  *     tags: [Account]
+ *     deprecated: true
  *     summary: Request a free API key (discontinued)
  *     description: Free accounts have been discontinued. Use the demo endpoints or upgrade to Pro.
  *     responses: