feat: monitors worker, auth register, scan polish

Add background DNS/SSL monitor checks, registration API, smoother SSE
with stage labels, improved CDN/WAF detection, light-theme UI polish,
and README roadmap updates.

Co-authored-by: Cursor <cursoragent@cursor.com>

prisma/schema.prisma

@@ -43,6 +43,7 @@ model Scan {
   domain    String
   status    ScanStatus @default(PENDING)
   progress  Int        @default(0)
+  stage     String?
   result    Json?
   error     String?
   userId    String?