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

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "docfast-api",
-  "version": "0.4.1",
+  "version": "0.4.3",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "docfast-api",
-      "version": "0.4.1",
+      "version": "0.4.3",
       "dependencies": {
         "compression": "^1.8.1",
         "express": "^4.21.0",