Add rate limit headers to OpenAPI generation script

- Update generate-openapi.mjs to include header components
- Ensure public/openapi.json has rate limit headers
- Update rate limits description in generation script

scripts/generate-openapi.mjs

@@ -29,6 +29,8 @@ Try the API without signing up! Demo endpoints are public (no API key needed) bu
 - Demo: 5 PDFs/hour per IP (watermarked)
 - Pro tier: 5,000 PDFs/month, 30 req/min
 
+All rate-limited endpoints return \`X-RateLimit-Limit\`, \`X-RateLimit-Remaining\`, and \`X-RateLimit-Reset\` headers. On \`429\`, a \`Retry-After\` header indicates seconds until the next allowed request.
+
 ## Getting Started
 1. Try the demo at \`POST /v1/demo/html\` — no signup needed
 2. Subscribe to Pro at [docfast.dev](https://docfast.dev/#pricing) for clean PDFs
@@ -64,6 +66,36 @@ Try the API without signing up! Demo endpoints are public (no API key needed) bu
           description: 'API key via X-API-Key header'
         }
       },
+      headers: {
+        'X-RateLimit-Limit': {
+          description: 'The maximum number of requests allowed in the current time window',
+          schema: {
+            type: 'integer',
+            example: 30
+          }
+        },
+        'X-RateLimit-Remaining': {
+          description: 'The number of requests remaining in the current time window',
+          schema: {
+            type: 'integer',
+            example: 29
+          }
+        },
+        'X-RateLimit-Reset': {
+          description: 'Unix timestamp (seconds since epoch) when the rate limit window resets',
+          schema: {
+            type: 'integer',
+            example: 1679875200
+          }
+        },
+        'Retry-After': {
+          description: 'Number of seconds to wait before retrying the request (returned on 429 responses)',
+          schema: {
+            type: 'integer',
+            example: 60
+          }
+        }
+      },
       schemas: {
         PdfOptions: {
           type: 'object',